patch for libgcrypt 1.2.0 on pthreads

[Florian Weimer]

* Werner Koch:

> malloc returns void* and that type must be assignable to any other
> pointer object.  Thus this patch does not make sense for C.

The patch is for a header file, which probably should be made
C++-friendly if it's supposed to be a public interface.  (But at least
in libgcrypt 1.1.90, the offending parts are missing from that file.)