shell: Use proper path to include girepository headers

shell: Use proper path to include girepository headers

girepository-2.0 headers are inside ${PREFIX}/include/glib-2.0/girepository
and the pkg-config file provides the glib-2.0 subpath as include
directory, so we are required to use the full header path, or we may not
be able to find them.

This is triggered a build failure in debian (with clean setup where no
libgirepository-1.0 was installed):

  ../src/shell-global.c:19:10: fatal error: girepository.h: No such file or
  directory
     19 | #include <girepository.h>

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3837>