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/nmi.c

@@ -5,8 +5,8 @@
 #include <stdint.h>
 #include <string.h>
 #include <wchar.h>
-#include "86box_io.h"
-#include "nmi.h"
+#include <86box/io.h>
+#include <86box/nmi.h>
 
 
 int nmi_mask;