hsl: BTS support

This patch adds the BTS support for the hsl driver.

It includes two examples under the tests/ directory.
diff --git a/include/internal.h b/include/internal.h
index a1356ad..6969748 100644
--- a/include/internal.h
+++ b/include/internal.h
@@ -1,6 +1,8 @@
 #ifndef _INTERNAL_H_
 #define _INTERNAL_H_
 
+#include <stdint.h>
+
 /* talloc context for libosmo-abis. */
 extern void *libosmo_abis_ctx;
 
@@ -10,5 +12,7 @@
 /* hsl requires these functions defined in ipaccess driver. */
 struct msgb;
 void ipaccess_prepend_header(struct msgb *msg, int proto);
+struct msgb *ipa_msg_alloc(int headroom);
+void ipa_msg_push_header(struct msgb *msg, uint8_t proto);
 
 #endif