Fix build with non-standard glib location

pkg-config --cflags glib-2.0 already has glib-2.0/ in the include path (and does not have parent directory of glib-2.0/ there):

On the host with non-standard glib location:

build-opensuse-10:~ # pkg-config --cflags glib-2.0
-I/opt/gnome/include/glib-2.0 -I/opt/gnome/lib64/glib-2.0/include  

On the host with standard glib location:

build-opensuse-11:~ # pkg-config --cflags glib-2.0
-I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include