missing.h: Explicitly check for IFLA_BRPORT_PROXYARP

RHEL explicitly disables IFLA_BRPORT_PROXYARP by renaming the enum value.

In order to support unpatched builds, we have two options:

a) redefine the enum value through missing.h and ignore the fact that it
   is really unsupported, or

b) omit that enum value on rtnl_prot_info_bridge_port_types[]

As we are not actually using this netlink type anywhere, and because it
is only hooked up for the sake of completeness, this patch opts for the
former.

configure.ac

@@ -323,6 +323,7 @@ AC_CHECK_DECLS([IFLA_INET6_ADDR_GEN_MODE,
                 IFLA_IPTUN_ENCAP_DPORT,
                 IFLA_GRE_ENCAP_DPORT,
                 IFLA_BRIDGE_VLAN_INFO,
+                IFLA_BRPORT_PROXYARP,
                 IFLA_BRPORT_LEARNING_SYNC,
                 IFLA_BR_PRIORITY,
                 NDA_IFINDEX,

src/basic/missing.h

@@ -883,13 +883,16 @@ static inline int setns(int fd, int nstype) {
 #define IFLA_BRPORT_FAST_LEAVE 7
 #define IFLA_BRPORT_LEARNING 8
 #define IFLA_BRPORT_UNICAST_FLOOD 9
-#define IFLA_BRPORT_PROXYARP 10
 #define IFLA_BRPORT_LEARNING_SYNC 11
 #define __IFLA_BRPORT_MAX 12
 
 #define IFLA_BRPORT_MAX (__IFLA_BRPORT_MAX - 1)
 #endif
 
+#if !HAVE_DECL_IFLA_BRPORT_PROXYARP
+#define IFLA_BRPORT_PROXYARP 10
+#endif
+
 #if !HAVE_DECL_NDA_IFINDEX
 #define NDA_UNSPEC 0
 #define NDA_DST 1