japanese/mutt-devel: MAILDIR_HEADER_CACHE implies IMAP_HEADER_CACHE

Before r316624 the port had

.if defined(WITH_MUTT_MAILDIR_HEADER_CACHE)
WITH_MUTT_IMAP_HEADER_CACHE= yes
.endif

which was wrongly converted to the current construct. It looks
like MAILDIR_HEADER_CACHE=on was meant to imply IMAP_HEADER_CACHE=on.