qt: Align license of some Makefiles with license of other Makefiles

* lang/qt/doc/Makefile.am, lang/qt/tests/Makefile.am: Change license
from LGPL to GPL.

Both files are part of QGpgME (as lang/qt/Makefile.am and
lang/qt/src/Makefile.am). Hence, they should be licensed as other parts
of QGpgME.