Start setting QT_DEFINES again.

The machinery in bsd.qt.mk's qt-post-install target does not seem to account
for the case of a module no longer defining QT_DEFINES: the lines in
qconfig-modules.h including said module's qconfig-<module>.h will remain.

We did that to qt5-multimedia in r458338, and it results in build errors if
qt5-multimedia had been previously installed. Set QT_DEFINES again to a dummy
value until we figure out a proper solution.

PR: 225100