include: Add proper predeclaration for struct in abis_rsl.h

We use them, so declare them. (avoid including all gsm_data.h)

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/openbsc/include/openbsc/abis_rsl.h b/openbsc/include/openbsc/abis_rsl.h
index 0b732d9..309b5c6 100644
--- a/openbsc/include/openbsc/abis_rsl.h
+++ b/openbsc/include/openbsc/abis_rsl.h
@@ -27,6 +27,11 @@
 
 #include <osmocore/msgb.h>
 
+struct gsm_bts;
+struct gsm_lchan;
+struct gsm_subscriber;
+
+
 int rsl_bcch_info(struct gsm_bts_trx *trx, u_int8_t type,
 		  const u_int8_t *data, int len);
 int rsl_sacch_filling(struct gsm_bts_trx *trx, u_int8_t type,