Mute sign compare warning by casting rc to u_int to match nbindings' type

rc cannot be negative -- that was already tested for earlier on in
the function

MFC after: 1 week
Reported by: clang, gcc
Sponsored by: EMC / Isilon Storage Division

usr.sbin/bsnmpd/tools/bsnmptools/bsnmpget.c

@@ -500,7 +500,7 @@ snmptool_walk(struct snmp_toolinfo *snmptoolctx)
 			outputs += rc;
 			snmp_pdu_free(&resp);
 
-			if (rc < resp.nbindings)
+			if ((u_int)rc < resp.nbindings)
 				break;
 
 			snmpwalk_nextpdu_create(op,