recheck_focus doesn't need to return anything

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

src/compton.h

@@ -42,7 +42,7 @@ uint32_t determine_evmask(session_t *ps, xcb_window_t wid, win_evmode_t mode);
 
 xcb_window_t find_client_win(session_t *ps, xcb_window_t w);
 
-struct managed_win *recheck_focus(session_t *ps);
+void recheck_focus(session_t *ps);
 
 /// Handle configure event of a root window
 void configure_root(session_t *ps, int width, int height);