Fix compilation on MS-Windows

* src/callproc.c (emacs_spawn) <fork_done>: Define the label only
if USABLE_POSIX_SPAWN is defined, to avoid a compiler warning.

(cherry picked from commit a8fc080851)

src/callproc.c

@@ -1602,7 +1602,9 @@ emacs_spawn (pid_t *newpid, int std_in, int std_out, int std_err,
 
   vfork_error = pid < 0 ? errno : 0;
 
+#if USABLE_POSIX_SPAWN
  fork_done:
+#endif
   if (pid < 0)
     {
       eassert (0 < vfork_error);