vapi: Add glib-unix.h functions to glib-2.0.vapi

To bind the things in glib-unix.h. Creating a separate glib-unix-2.0.vapi is
unneccessary because these functions are present in libglib-2.0.so itself,
rather than a separate library. They just require the glib-unix.h header to
be included.

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=667894