Correctly support IPv6 in Solaris (#17)

I can't see how adding one more OS to the manual list can hurt for now, so in goes the PR.  Thanks.

miltermodule.c

@@ -282,7 +282,7 @@ $ python setup.py help
  * published.  Unfortunately I know of no good way to do this
  * other than with OS-specific tests.
  */
-#if defined(__FreeBSD__) || defined(__linux__)
+#if defined(__FreeBSD__) || defined(__linux__) || defined(__sun__)
 #define HAVE_IPV6_RFC2553
 #include <arpa/inet.h>
 #endif