Make Requires private in pkg-config files

Make Requires private in pkg-config files

Putting stuff in Requires means applications using
this library will link against that when it might
not be needed at all because they use none of that
libraries symbols directly.

If only the cflags is needed (because public headers
includes headers from that package) they
can be private to avoid uselessly linking.

See #2 in http://people.freedesktop.org/~dbn/pkg-config-guide.html#faq

https://bugzilla.gnome.org/show_bug.cgi?id=685477