[gutil] fix compilation warning on macOS

This patch fixes compilation warning on contemporary versions of Xcode
(starting at least with Xcode 12.4):

  In file included from src/kudu/util/status.h:30:
  In file included from src/kudu/gutil/macros.h:14:
  src/kudu/gutil/port.h:237:9: warning: 'MSG_NOSIGNAL' macro redefined [-Wmacro-redefined]
  #define MSG_NOSIGNAL 0
	  ^
  <SDK>/usr/include/sys/socket.h:588:9: note: previous definition is here

Change-Id: Ie0ec3876ccb8dcc6f67725cfb4a7380b092e0384
Reviewed-on: http://gerrit.cloudera.org:8080/19200
Tested-by: Kudu Jenkins
Reviewed-by: Yifan Zhang <chinazhangyifan@163.com>
1 file changed