Move internal/private dependencies to Requires.private

Move internal/private dependencies to Requires.private

Program using the xcb sub-modules has indirect compile and runtime
dependency of core xcb. To ensure this out we currently list xcb in
the Requires field of the pkg-config files. While this provides all
the required dependencies for successful compilation this causes
over-linking and hides potential linking miss-use against the xcb modules.

By moving to Requires.private we retain the compilation and runtime
compatibility and avoids any runtime problems.

Cc: Keith Packard <keithp@keithp.com>
Cc: Alan Coopersmith <alan.coopersmith@oracle.com>
References: http://people.freedesktop.org/~dbn/pkg-config-guide.html#faq
References: https://wiki.mageia.org/en/Overlinking_issues_in_packaging
References: http://err.no/personal/blog/2008/Mar/25
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Uli Schlachter <psychon@znc.in>