Don't rebuild ks_tables.h if nothing changed.

ks_tables.h is always considered out of date due to the forced rebuild
of the makekeys util. This means the file is also rebuilt during 'make
install', which is usually performed as root, which can to lead
permission problems later on.

Signed-off-by: Martin Natano <natano@natano.net>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>