stb_leakcheck: Make stb_leakcheck_free actually free.

Fixes issue #307.

stb_leakcheck.h

@@ -59,6 +59,7 @@ void stb_leakcheck_free(void *ptr)
       if (mi->next)
          mi->next->prev = mi->prev;
       #endif
+      free(ptr);
    }
 }