require postfix is installed before installing postfix-pcre (#4223)

Change-Id: I547b99becb8b16fec0ac89f06fb6d833cbde3c2b
certtool-postfix-gendh attempted before postfix is installed (Bug #4340)

Change-Id: I4ffb5b9203741d1152dfd93ef9ecc45f6a6088d4