Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

key_mapping_haiku.cpp

@@ -30,8 +30,8 @@
 
 #include <InterfaceDefs.h>
 
+#include "core/os/keyboard.h"
 #include "key_mapping_haiku.h"
-#include "os/keyboard.h"
 
 struct _HaikuTranslatePair {
 	unsigned int keysym;