Mute -Wstrlcpy-strlcat-size warning by using nitems with the size of the buffer

This is a no-op as the malloc above set the size of the buffer to the size used
below, but this keeps things consistent in case the malloc call changes somehow.

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

usr.sbin/bsnmpd/tools/libbsnmptools/bsnmpimport.c

@@ -774,7 +774,7 @@ snmp_import_object(struct snmp_toolinfo *snmptoolctx)
 		return (-1);
 	}
 
-	strlcpy(string, nexttok, strlen(nexttok) + 1);
+	strlcpy(string, nexttok, nitems(string));
 	oid2str->string = string;
 	oid2str->strlen = strlen(nexttok);