Turns out the fix in commit 93555577 wasn't enough, when using glib as
subproject and the parent project uses only libgio_dep, and include
<gi18n.h>, it won't find libintl.h because it's in the
include_directories of libglib_dep. Fix that by declaring dependencies
explicitly, which is the right thing to do since glib and gobject are
public dependencies of gio. That reflects what we do for the pkg-config
file as well.