sed(1): Suppress implicit-fallthrough.

Apparently some tools are not able to determine if all the cases of a
switch are covered. Make use of the attribute for cases like this.

Hinted by:	DragonFlyBSD GGC8 (but fixed differently)

CID:	976552

usr.bin/sed/main.c

@@ -250,6 +250,8 @@ again:
 			s = script->s;
 			state = ST_STRING;
 			goto again;
+		default:
+			__unreachable();
 		}
 	case ST_FILE:
 		if ((p = fgets(buf, n, f)) != NULL) {