missing.h: Explicitly check for IFLA_BRPORT_PROXYARP

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.