tcp-wrappers: fix parallel builds

Inparallel builds make can try to rebuild a object file while cflags
file is not yet built. Fix that by providing an explicit cflags
dependency on config-check.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>