gallium: fix a warning

On some platforms (like Win64), unsigned long is 32-bit, so the first
cast doesn't do anything, and the compiler complains about an implicit
cast to a smaller type. So let's cast to an uintptr_t instead first,
as that's large enough on all platforms.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/util/u_debug_flush.c

@@ -111,7 +111,7 @@ debug_flush_pointer_compare(void *key1, void *key2)
 static unsigned
 debug_flush_pointer_hash(void *key)
 {
-   return (unsigned) (unsigned long) key;
+   return (unsigned) (uintptr_t) key;
 }
 
 struct debug_flush_buf *