Add <netinet/in.h> include on FreeBSD

FreeBSD does not include headers recursively, so we need to include it
explicitly at least for using IPPROTO_UDP macro.

Thanks Po-Chuan Hsieh (@sunpoet) for the fix proposal (PR #3739).

Fixes #3677.

src/say.c

@@ -37,6 +37,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <netdb.h>
+#include <netinet/in.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <fcntl.h>