kde.mk, qt.mk: Prefix component variables to avoid clashes.

r496074 was necessary because both kde.mk and qt.mk were setting wayland_LIB
and wayland_PORT, and qt.mk overrode the version in kde.mk.

Follow what we do in pyqt.mk and prefix the component variables to avoid future
clashes.

Reviewed by: tcberner
Differential Revision: https://reviews.freebsd.org/D19631