Issue #8407: Use an explicit cast for FreeBSD

pthread_t is a pointer, not an integer, on FreeBSD. It should fix the following
gcc warning:

passing argument 1 of ‘pthread_kill’ makes pointer from integer without a cast

Modules/signalmodule.c

@@ -688,7 +688,7 @@ signal_pthread_kill(PyObject *self, PyObject *args)
     if (!PyArg_ParseTuple(args, "li:pthread_kill", &tid, &signum))
         return NULL;
 
-    err = pthread_kill(tid, signum);
+    err = pthread_kill((pthread_t)tid, signum);
     if (err != 0) {
         errno = err;
         PyErr_SetFromErrno(PyExc_OSError);