Always compile `_starpu_free_unref'.

The `#ifdef' would compile it only when `-fplugin=.../starpu.so' is
used when building StarPU itself, which is not what we want.

src/util/malloc.c

@@ -239,11 +239,9 @@ int starpu_free(void *A)
 	return 0;
 }
 
-#ifdef STARPU_GCC_PLUGIN
 /* Internal convenience function, used by code generated by the GCC
  * plug-in.  */
 void _starpu_free_unref(void *p)
 {
 	(void)starpu_free(* (void **)p);
 }
-#endif