Let old/new backends co-exist

Now both the old and the new backends are compiled in, the user can
choose which one to use with a command line switch.

Lower the barrier for testing.

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

src/compton.h

@@ -13,6 +13,7 @@
 
 #include <X11/Xutil.h>
 #include "common.h"
+#include "backend/backend.h"
 #include "win.h"
 #include "x.h"
 #include "c2.h"