libcgroup: Add pkg-config data

libcgroup: Add pkg-config data

libcgroup was not making an entry into the pkg-config database, which
makes life easier when using multiple libraries. Add the needed data
in order to be registered with the pkg-config database on installation.

Signed-off-by: Dhaval Giani <dhaval.giani@gmail.com>
Reviewed-by: Jan Safranek <jsafrane@redhat.com>
Acked-by: Lennart Poettering <lennnart@poettering.net>