Fix compiler warning

Apple clang version 12.0.5 (clang-1205.0.22.9)
Target: arm64-apple-darwin20.4.0

Warning appeared after upgrade to Xcode 12.5 in several files,
example:

In file included from Fl_Scroll.cxx:19:
../FL/Fl_Scroll.H:102:17: warning: anonymous non-C-compatible type given name for linkage purposes by typedef declaration; add a tag name here [-Wnon-c-typedef-for-linkage]
  typedef struct {
                ^
                 ScrollInfo
../FL/Fl_Scroll.H:104:5: note: type is not C-compatible due to this member declaration
    typedef struct { int x,y,w,h; } Fl_Region_XYWH;
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../FL/Fl_Scroll.H:128:5: note: type is given name 'ScrollInfo' for linkage purposes by this typedef declaration
  } ScrollInfo;
    ^

FL/Fl_Scroll.H

@@ -1,19 +1,17 @@
 //
-// "$Id$"
-//
 // Scroll header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2015 by Bill Spitzak and others.
+// Copyright 1998-2021 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
 // file is missing or damaged, see the license at:
 //
-//     http://www.fltk.org/COPYING.php
+//     https://www.fltk.org/COPYING.php
 //
-// Please report all bugs and problems on the following page:
+// Please see the following page on how to report bugs and issues:
 //
-//     http://www.fltk.org/str.php
+//     https://www.fltk.org/bugs.php
 //
 
 /* \file
@@ -99,15 +97,10 @@ protected:	// NEW (STR#1895)
 #else
 private:	// OLD
 #endif
-  /**
+
-    Structure to manage scrollbar and widget interior sizes.
-    This is filled out by recalc_scrollbars() for use in calculations
-    that need to know the visible scroll area size, etc.
-    \note Availability in FLTK_ABI_VERSION 10303 or higher.
-  */
-  typedef struct {
   /// A local struct to manage a region defined by xywh
   typedef struct { int x,y,w,h; } Fl_Region_XYWH;
+
   /// A local struct to manage a region defined by left/right/top/bottom
   typedef struct {
     int l;		///< (l)eft "x" position, aka x1
@@ -115,6 +108,7 @@ private:	// OLD
     int t;		///< (t)op "y" position, aka y1
     int b;		///< (b)ottom "y" position, aka y2
   } Fl_Region_LRTB;
+
   /// A local struct to manage a scrollbar's xywh region and tab values
   typedef struct {
     int x,y,w,h;
@@ -123,6 +117,14 @@ private:	// OLD
     int first;		///< scrollbar tab's "number of first line"
     int total;		///< scrollbar tab's "total number of lines"
   } Fl_Scrollbar_Data;
+
+  /**
+    Structure to manage scrollbar and widget interior sizes.
+    This is filled out by recalc_scrollbars() for use in calculations
+    that need to know the visible scroll area size, etc.
+    \note Availability in FLTK_ABI_VERSION 10303 or higher.
+  */
+  typedef struct {
     int scrollsize;		///< the effective scrollbar thickness (local or global)
     Fl_Region_XYWH innerbox;	///< widget's inner box, excluding scrollbars
     Fl_Region_XYWH innerchild;	///< widget's inner box, including scrollbars
@@ -132,6 +134,7 @@ private:	// OLD
     Fl_Scrollbar_Data hscroll;	///< horizontal scrollbar region + values
     Fl_Scrollbar_Data vscroll;	///< vertical scrollbar region + values
   } ScrollInfo;
+
   void recalc_scrollbars(ScrollInfo &si);
 
 protected:
@@ -203,7 +206,3 @@ public:
 };
 
 #endif
-
-//
-// End of "$Id$".
-//