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/snd_opl_backend.h

@@ -0,0 +1,17 @@
+/* Copyright holders: Sarah Walker, SA1988
+   see COPYING for more details
+*/
+#ifdef __cplusplus
+extern "C" {
+#endif
+        void opl_init(void (*timer_callback)(void *param, int timer, uint64_t period), void *timer_param, int nr, int is_opl3);
+        void opl_write(int nr, uint16_t addr, uint8_t val);
+        uint8_t opl_read(int nr, uint16_t addr);
+        void opl_timer_over(int nr, int timer);
+        void opl2_update(int nr, int32_t *buffer, int samples);
+        void opl3_update(int nr, int32_t *buffer, int samples);
+
+	extern int opl_type;
+#ifdef __cplusplus
+}
+#endif