ip: Fix compilation break on old systems

As was reported [1], the iproute2 fails to compile on old systems,
in Cong's case, it was Fedora 19, in our case it was RedHat 7.2, which
failed with the following errors during compilation:

ipxfrm.c: In function ‘xfrm_selector_print’:
ipxfrm.c:479:7: error: ‘IPPROTO_MH’ undeclared (first use in this
function)
  case IPPROTO_MH:
       ^
ipxfrm.c:479:7: note: each undeclared identifier is reported only once
for each function it appears in
ipxfrm.c: In function ‘xfrm_selector_upspec_parse’:
ipxfrm.c:1345:8: error: ‘IPPROTO_MH’ undeclared (first use in this
function)
   case IPPROTO_MH:
        ^                                                                                                                                                            make[1]: *** [ipxfrm.o] Error 1

The reason to it is the order of headers files. The IPPROTO_MH field is
set in kernel's UAPI header file (in6.h), but only in case
__UAPI_DEF_IPPROTO_V6 is set before. That define comes from other kernel's
header file (libc-compat.h) and is set in case there are no previous
libc relevant declarations.

In ip code, the include of <netdb.h> causes to indirect inclusion of
<netinet/in.h> and it sets __UAPI_DEF_IPPROTO_V6 to be zero and prevents from
IPPROTO_MH declaration.

This patch takes the simplest possible approach to fix the compilation
error by checking if IPPROTO_MH was defined before and in case it
wasn't, it defines it to be the same as in the kernel.

[1] https://www.spinics.net/lists/netdev/msg463980.html

Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Riad Abo Raed <riada@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>

ip/xfrm.h

@@ -30,6 +30,10 @@
 #include <linux/xfrm.h>
 #include <linux/ipsec.h>
 
+#ifndef IPPROTO_MH
+#define IPPROTO_MH              135
+#endif
+
 #define XFRMS_RTA(x)  ((struct rtattr*)(((char*)(x)) + NLMSG_ALIGN(sizeof(struct xfrm_usersa_info))))
 #define XFRMS_PAYLOAD(n) NLMSG_PAYLOAD(n,sizeof(struct xfrm_usersa_info))