Move all include files to src/include

- 86Box's own headers go to /86box
- munt's public interface goes to /mt32emu
- all slirp headers go to /slirp (might want to consider using only its public inteface)
- single file headers from other projects go in include root

src/win/win_joystick.cpp

@@ -23,11 +23,11 @@
 #include <stdint.h>
 #include <stdio.h>
 #define HAVE_STDARG_H
-#include "86box.h"
-#include "device.h"
-#include "plat.h"
-#include "gameport.h"
-#include "win.h"
+#include <86box/86box.h>
+#include <86box/device.h>
+#include <86box/plat.h>
+#include <86box/gameport.h>
+#include <86box/win.h>
 
 
 plat_joystick_t plat_joystick_state[MAX_PLAT_JOYSTICKS];