qt,tests: Fix build in source directory, part 2

* lang/qt/tests/Makefile.am (AM_CPPFLAGS): Include Qt binding sources
before C++ binding sources and C sources.

This fixes the problem that the debug.h in the C sources was found
before the one in the Qt bindings.

Commit d23528cadf4284f508bc4ba05e1b5c14b46bd354 fixed it for Qt5, we
need Qt6 as well.

• GnuPG-bug-id: T6673