meson: Don't use outdated 'fallback:' kwarg to dependency()

The mapping is done using the wrap file now. Also update the libffi
and pcre2 subprojects at the same time.

Reported by Benjamin Gilbert at
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3172

subprojects/libffi.wrap

@@ -3,3 +3,6 @@ directory=libffi
 url=https://gitlab.freedesktop.org/gstreamer/meson-ports/libffi.git
 revision=meson
 depth=1
+
+[provide]
+libffi = ffi_dep