qt,build: Install headers for Qt 5 and Qt 6 in different locations

* lang/qt/src/Makefile.am (includeprefix): New.
(qgpgmeincludedir, camelcaseincludedir): Add $(includeprefix).
(QGpgmeConfig.cmake, QGpgmeQt6Config.cmake): Add $(includeprefix) to
replacement for @resolved_includedir@.

This installs the headers for Qt 5 in $(includedir)/qgpgme-qt5 and the
headers for Qt 6 in $(includedir)/qgpgme-qt6. For users of the CMake
files the change will be transparent, i.e. they don't have to do
anything. Packagers will have to change their scripts but in the end
this change will make their lifes easier.

• GnuPG-bug-id: T7161