gnome.mk: fix usage of GLIB_SCHEMAS in five ports

Glib schema files should be registered using the GLIB_SCHEMAS
variable provided by gnome.mk. This will add them and additionally
the required glib-schemas keyword to the plist.

The following ports were touched:

• deskutils/foliate
• deskutils/gnome-tweaks
• deskutils/maliit-keyboard
• games/swell-foop
• textproc/ibus-m17n