gcc: Fix typo when checking for HAVE_DECL_PTR_DEREFS_MAY_ALIAS_P.

* gcc-plugin/src/starpu.c (plugin_init): Check whether
  HAVE_DECL_PTR_DEREFS_MAY_ALIAS_P is zero, not whether it's defined.

gcc-plugin/src/starpu.c

@@ -3431,7 +3431,7 @@ plugin_init (struct plugin_name_args *plugin_info,
   register_callback (plugin_name, PLUGIN_PASS_MANAGER_SETUP,
 		     NULL, &pass_info);
 
-#ifdef HAVE_DECL_PTR_DEREFS_MAY_ALIAS_P
+#if HAVE_DECL_PTR_DEREFS_MAY_ALIAS_P
   /* This warning pass is only available when `ptr_derefs_may_alias_p' is
      available, with GCC >= 4.6.  */