glw: Mark all extern symbols GLAPI to regain default visibility (#31294)

glw: Mark all extern symbols GLAPI to regain default visibility (#31294)

Since switching to hidden visibility on gcc, GLw apps were failing to
link. Use the GLAPI definition to use default visibility where necessary.

$ nm lib/libGLw.so | grep DrawingArea
0000000000004020 T GLwCreateMDrawingArea
0000000000003430 T GLwDrawingAreaMakeCurrent
0000000000003410 T GLwDrawingAreaSwapBuffers
0000000000204c60 D glwDrawingAreaClassRec
0000000000204d48 D glwDrawingAreaWidgetClass
00000000002053c0 D glwMDrawingAreaClassRec
00000000002054e0 D glwMDrawingAreaWidgetClass

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>
Tested-by: justin <jlec@gentoo.org>