(my_systemv): cleanup unreachable-code warning.

Compiler with my_exit() 'noreturn' knowledge will complain about
never reachable break statement.

Reported by clang compiler.

(Maybe there is a better code sequence)

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

lib/utilunix.c

@@ -452,7 +452,7 @@ my_systemv (const char *command, char *const argv[])
             execvp (command, argv);
             my_exit (127);      /* Exec error */
         }
-        break;
+        /* no break here, or unreachable-code warning by no returning my_exit() */
     default:
         status = 0;
         break;