Fix the .pc file installed by devel/qt5-designer.
Followup to r466825 (fix which causes failure in devel/py-qt5-designer)
and r467015 (which reverted the fix). Change only the installed .pc
file, not the .pro file (which has knock-on effects for ports using
qmake, like the py-qt5-designer* ones).
Tested this time with
poudriere bulk ... -i -C
x11-toolkits/qt5-uiplugin devel/qt5-designer devel/py-qt5-designer devel/py-qt5-designerplugin devel/qscintilla2-designerplugin-qt5
Approved by: tcberner (mentor, implicit)