wm: fix build on 32-bit systems

Force uint64_t to be aligned to 8-bytes even on 32-bit systems, so we
don't need a special case for them.

Fixes #1346

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/wm/wm.h

@@ -63,7 +63,7 @@ enum wm_tree_change_type {
 typedef struct wm_treeid {
 	/// The generation of the window ID. This is used to detect if the window ID is
 	/// reused. Inherited from the wm_tree at cr
-	uint64_t gen;
+	alignas(8) uint64_t gen;
 	/// The X window ID.
 	xcb_window_t x;