Silence compiler warnings related to not using assuan_fd_t.

* common/call-gpg.c (start_gpg): Use assuan_fd_t.  Note that the
declaration was already fixed by a previous change.
* dirmngr/server.c (cmd_getinfo): Use assuan_fd_t.

Note that this matters only for Windows and it does not harm as long
as we can only build for 32 bit Windows withsizeof(int)==sizeof(void*).

• Signed-off-by: Werner Koch <wk@gnupg.org>