Add #ifdef INET6 around declaration of nbuf.

It was reported that without #ifdef INET6 around the declaration of "nbuf",
a build would report an unused variable. For some reason, I didn't see that
warning when I did a build, but it seems reasonable to add these #ifdef INET6's.

Submitted by:	[email protected]
MFC after:	1 week

usr.sbin/nfsdumpstate/nfsdumpstate.c

@@ -121,7 +121,9 @@ dump_openstate(void)
 {
 	struct nfsd_dumplist dumplist;
 	int cnt, i;
+#ifdef INET6
 	char nbuf[INET6_ADDRSTRLEN];
+#endif
 
 	dumplist.ndl_size = DUMPSIZE;
 	dumplist.ndl_list = (void *)dp;
@@ -176,7 +178,9 @@ dump_lockstate(char *fname)
 {
 	struct nfsd_dumplocklist dumplocklist;
 	int cnt, i;
+#ifdef INET6
 	char nbuf[INET6_ADDRSTRLEN];
+#endif
 
 	dumplocklist.ndllck_size = DUMPSIZE;
 	dumplocklist.ndllck_list = (void *)lp;