Make devel/qt5-help a build-time dependency.

It is only used to produce the .qch file, so it does not need to be present at
runtime. This turns kf5-e-c-m into a port that has no dependency on Qt5 (or
basically any other port), which makes it easier to depend on it.

Reviewed by: tcberner
MFH: 2017Q2
Differential Revision: https://reviews.freebsd.org/D10234