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/prt_devs.h

@@ -0,0 +1,3 @@
+extern const lpt_device_t	lpt_prt_text_device;
+extern const lpt_device_t	lpt_prt_escp_device;
+extern const lpt_device_t	lpt_prt_ps_device;