Fix compiler warnings
Fix compiler warnings

Move multiple comment block with SECTION:gxps-version
after the header guard.

- Fixes the following warning (6x):
  libgxps/gxps-version.h:19: Warning: GXPS: multiple comment blocks
  documenting 'SECTION:gxps-version:' identifier
  (already seen at gxps-version.h:19).

GNOME/libgxps,GNOME/libgxps,GNOME/libgxps