Fixup commit 604cd4a for OpenIndiana.

Fixup commit 604cd4a for OpenIndiana.

SunOS openindiana 5.11 illumos-755ccbcfa2 i86pc i386 i86pc
gcc (OpenIndiana 10.3.0-oi-0) 10.3.0

As it turns out, in OpenIndiana 2021.04 ND_UNREACHABLE is one of the
neighbour discovery states in <net/if.h>:

In file included from ./extract.h:40,
                 from ./addrtoname.h:22,
                 from ./addrtoname.c:112:
./diag-control.h:181: warning: "ND_UNREACHABLE" redefined

In file included from ./addrtoname.c:87:
/usr/include/net/if.h:307: note: this is the location of the previous
  definition

In file included from ./extract.h:40,
                 from ./addrtoname.h:22,
                 from ./print-sll.c:36:
./diag-control.h:181: warning: "ND_UNREACHABLE" redefined

In file included from ./print-sll.c:29:
/usr/include/net/if.h:307: note: this is the location of the previous
  definition