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/net_3c503.h

@@ -0,0 +1,7 @@
+#ifndef NET_3C503_H
+# define NET_3C503_H
+
+extern const device_t	threec503_device;
+
+
+#endif	/*NET_3C503_H*/