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/include/86box/midi_input.h

@@ -0,0 +1 @@
+extern const device_t midi_input_device;