mingw fix: avoid "unused var" warning.

This is a backport of 19615bce64 to
fix bug 22838.

src/test/test_switch_id.c

@@ -79,6 +79,7 @@ main(int argc, char **argv)
 #if defined(_WIN32)
   (void) argc;
   (void) argv;
+  (void) which_test;
 
   fprintf(stderr, "This test is not supported on your OS.\n");
   return 77;