Fix GCC 7 -Wimplicit-fallthrough warnings.

src/luajit.c

@@ -419,6 +419,7 @@ static int collectargs(char **argv, int *flags)
       break;
     case 'e':
       *flags |= FLAGS_EXEC;
+      /* fallthrough */
     case 'j':  /* LuaJIT extension */
     case 'l':
       *flags |= FLAGS_OPTION;