Only define USE_LDCONFIG in devel/qt5-core

All the qt5-ports share the same library directory. devel/qt5-core is dependet on
by all others.
So there is no need to add identical entries to libdata/ldconfig, and restart the
ldconfig service on every pkg change of those ports.

Reported by: bapt
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D27224