(my_systemv): cleanup implicit-fallthrough warning.

Found by GCC 7.2.0

utilunix.c: In function 'my_systemv':
utilunix.c:455:13: error: this statement may fall through [-Werror=implicit-fallthrough=]
             my_exit (127);      /* Exec error */
             ^~~~~~~~~~~~~
utilunix.c:458:5: note: here
     default:
     ^~~~~~~

Signed-off-by: Andreas Mohr <and@gmx.li>
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

lib/utilunix.c

@@ -454,6 +454,7 @@ my_systemv (const char *command, char *const argv[])
             execvp (command, argv);
             my_exit (127);      /* Exec error */
         }
+        MC_FALLTHROUGH;
         /* no break here, or unreachable-code warning by no returning my_exit() */
     default:
         status = 0;