syscall: don't generate RTF_BITS constant on OS X Mavericks and beyond

LGTM=iant
R=iant, bradfitz
CC=golang-codereviews
https://golang.org/cl/80700044

src/pkg/syscall/mkerrors.sh

@@ -269,6 +269,7 @@ ccflags="$@"
 		$2 ~ /^(NETLINK|NLM|NLMSG|NLA|IFA|IFAN|RT|RTCF|RTN|RTPROT|RTNH|ARPHRD|ETH_P)_/ ||
 		$2 ~ /^SIOC/ ||
 		$2 ~ /^TIOC/ ||
+		$2 !~ "RTF_BITS" &&
 		$2 ~ /^(IFF|IFT|NET_RT|RTM|RTF|RTV|RTA|RTAX)_/ ||
 		$2 ~ /^BIOC/ ||
 		$2 ~ /^RUSAGE_(SELF|CHILDREN|THREAD)/ ||