Add patch for better thunderbird compat in GpgOL

* Makefile.am (EXTRA_DIST): Add it.
* patches/gpgol/
0001-Handle-protected-headers-also-on-multipart-parts.patch: New.

This is a patch from GpgOL 2.5 branch which became more important
since it was reported to me that recent thunderbird has mandatory
subject encryption.