utils: add libsss_child dependency to libsss_cert
Since the refactoring of the ssh responder to call p11_child to
validate certificates there is a dependency between libss_cert and
libsss_child. In some environments, e.g. gentoo or the OpenSUSE build
service, this dependency must be declared explicitly even if it is
resolved otherwise while linking the binaries.
Reviewed-by: Fabiano Fidêncio <email@example.com>