The intention of the change is to clean up unnecessary linking with
libffi and gmodule in g-i consumers as they typically don't use any
libffi/gmodule symbols.

Listing libffi and gmodule in pkg-config Requires meant that every
consumer that wanted to link with -lgirepository also got -lffi and
-lgmodule-2.0, which caused ld to emit unnecessary DT_NEEDED entries.

Moving the deps to Requires.private removes the internal dependencies
from 'pkg-config --libs', but keeps them in --libs --static output. The
change won't affect --cflags and the include directories necessary for
<ffi.h> will still be listed.

https://bugzilla.gnome.org/show_bug.cgi?id=674065