Install gpgme and libgpg-error to qt-install-dir

If user had enabled custom qt build (i.e. they set qt-install-dir to some normal path), and built the gpgme/libgpg-error, then gpgme/libgpg-error are installed in install-dir.

These modules are only needed when custom qt build is in use.

In situation when gpgme/libgpg-error are installed, and the user decided to disable custom qt build (i.e. set qt-install-dir to empty or comment it), then, as gpgme/libgpg-error are installed into install-dir, the kde modules may be linked to custom built gpgme/libgpg-error, and that is not needed.

This commit changes the install-dir of these two modules to qt-install-dir, so, the kde modules will be linked to gpgme/libgpg-error only when custom qt build is enabled.