pkgconfig: fix build with gcc-15

pkgconfig: fix build with gcc-15

* on hosts with gcc-15 or whenever glib PACKAGECONFIG isn't enabled
  and pkgconfig uses own old bundled glib

* fixes:
  http://errors.yoctoproject.org/Errors/Details/853015/
../../../git/glib/glib/goption.c:169:14: error: two or more data types in declaration specifiers
  169 |     gboolean bool;
      |              ^~~~
../../../git/glib/glib/goption.c:169:18: warning: declaration does not declare anything
  169 |     gboolean bool;
      |                  ^

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>