pimd: cast to sockaddr_in to sockaddr

While glibc seems to have something in the system headers that prevents
this from triggering a warning, FreeBSD doesn't.  Fix the warning.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/pimd/pim_igmpv3.c b/pimd/pim_igmpv3.c
index 3baddbf..3657f2f 100644
--- a/pimd/pim_igmpv3.c
+++ b/pimd/pim_igmpv3.c
@@ -1680,7 +1680,8 @@
 #endif
   tolen = sizeof(to);
 
-  sent = sendto(fd, query_buf, msg_size, MSG_DONTWAIT, &to, tolen);
+  sent = sendto(fd, query_buf, msg_size, MSG_DONTWAIT,
+                (struct sockaddr *)&to, tolen);
   if (sent != (ssize_t) msg_size) {
     int e = errno;
     char dst_str[100];