util: systemd: Define MSG_NOSIGNAL if needed

The symbol being missing has been reported as causing build
failures on OS X. If it's not already defined, define it to
zero so that it won't have any effect.

src/util/virsystemd.c

@@ -41,6 +41,10 @@
 
 VIR_LOG_INIT("util.systemd");
 
+#ifndef MSG_NOSIGNAL
+# define MSG_NOSIGNAL 0
+#endif
+
 static void virSystemdEscapeName(virBufferPtr buf,
                                  const char *name)
 {