Mono: add patch for bxc15969

Backport:
https://github.com/mono/mono/commit/79e0856ffda4c2566314ba31677fe55f2d7f
53f2

To fix a segfault on amd64

Upstream bug report:

https://bugzilla.xamarin.com/show_bug.cgi?id=15969
gtk-sharp3: disable docs

We don't really need documentation and while it works with Mono 3.2.3,
monodoc crashes building it using mono git master.

Note: the configure script will still print that documentation is being
built but it is not. A german will be poked about this in the near
future.