meson: Add schemasdir and giomoduledir to gio dependency
This allows applications to get their value regardless whether glib is a subproject or pkgconfig:
gio_dep = dependency('gio-2.0')
giomoduledir = gio_dep.get_variable('giomoduledir')
schemasdir = gio_dep.get_variable('schemasdir')
Edited by Xavier Claessens