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/sound/snd_gus.c

@@ -5,15 +5,15 @@
 #include <stdlib.h>
 #include <wchar.h>
 #define HAVE_STDARG_H
-#include "86box.h"
-#include "86box_io.h"
-#include "nmi.h"
-#include "pic.h"
-#include "dma.h"
-#include "timer.h"
-#include "device.h"
-#include "sound.h"
-#include "midi.h"
+#include <86box/86box.h>
+#include <86box/io.h>
+#include <86box/nmi.h>
+#include <86box/pic.h>
+#include <86box/dma.h>
+#include <86box/timer.h>
+#include <86box/device.h>
+#include <86box/sound.h>
+#include <86box/midi.h>
 
 enum
 {