Add wxGetFileDescriptor()

This function already existed internally, make it public as it can be
useful and will be used by wxFFile itself in the upcoming commit.

interface/wx/filefn.h

@@ -543,6 +543,21 @@ enum wxFileKind
 
 ///@}
 
+/** @addtogroup group_funcmacro_file */
+///@{
+/**
+    Returns the file descriptor for the given @a fp.
+
+    @param fp
+        A valid pointer to a @c FILE.
+    @returns
+        The file descriptor for the given @a fp, or -1 on error.
+
+    @since 3.3.4
+ */
+int wxGetFileDescriptor(FILE *fp);
+///@}
+
 /** @addtogroup group_funcmacro_file */
 ///@{
 /**