Remove conflicting FL_EXPORT from header file (#1357)

Note: we can't have FL_EXPORT together with an *implementation* in
a header file.

If compiled with FL_DLL defined, the error message would be:
  "Fl_Font_Descriptor::~Fl_Font_Descriptor()' definition is marked
   dllimport"

... with some build types using gcc (e.g. cross-compiling for Windows).

FL/Fl_Graphics_Driver.H

@@ -395,7 +395,7 @@ public:
   Fl_Font_Descriptor *next;
   Fl_Fontsize size; /**< font size */
   Fl_Font_Descriptor(const char* fontname, Fl_Fontsize size);
-  virtual FL_EXPORT ~Fl_Font_Descriptor() {}
+  virtual /* FL_EXPORT */ ~Fl_Font_Descriptor() {}
   int ascent, descent;
   unsigned int listbase;// base of display list, 0 = none
 };