Adjust patches for GnuPG 2.0.21.

* patches/gnupg2/02-allow-mark-trusted.patch: Move to ...
* patches/gnupg2-2.0.20/: new directory.
* Makefile.am (EXTRA_DIST): Adjust for this.

We only allow current versions of GnuPG to be build with the
installer, thus we explicitly support 2.0.20 but prepare for future
version by removing a patch which meanwhile has be integrated into
GnuPG 2.0 proper.