build,qt: Always compile the Qt 6 test application with -fpic

* m4/qt6.m4: Do not add -fpic to GPGME_QT6_CFLAGS. Add -fpic to CPPFLAGS
used for build test of simple Qt 6 application.

The pkgconfig files of Qt6Core do not contain the qt_config variable,
so that we cannot easily check whether Qt6 was compiled with pic. For
simplicity we always compile the test application with -fpic to avoid
a build failure if Qt6 was actually compiled with pic.
For the actual build of QGpgME libtool automatically uses -fPIC, so that
we don't have to add it to the GPGME_QT6_CFLAGS.