Fix host signal vs gdb signal mixup in gdb/darwin-nat.c

Fix host signal vs gdb signal mixup in gdb/darwin-nat.c

Building in C++ mode caught a bug here:

 .../src/gdb/darwin-nat.c: In function 'ptid_t darwin_decode_message(mach_msg_header_t*, darwin_thread_t**, inferior**, target_waitstatus*)':
 .../src/gdb/darwin-nat.c:1016:25: error: invalid conversion from 'int' to 'gdb_signal' [-fpermissive]
      status->value.sig = WTERMSIG (wstatus);
			  ^

gdb/ChangeLog:
2016-04-20  Pedro Alves  <palves@redhat.com>

	* darwin-nat.c (darwin_decode_message): Use gdb_signal_from_host.