misc: Include abis_rsl.h to remove some compiler warnings
This was introduced by a recent change to gsm_data.h to include
less header files. We really need to access the RSL information
here so it is fine to include the file.
diff --git a/openbsc/src/bsc_api.c b/openbsc/src/bsc_api.c
index 043006e..0a7c94b 100644
--- a/openbsc/src/bsc_api.c
+++ b/openbsc/src/bsc_api.c
@@ -25,6 +25,7 @@
#include <openbsc/bsc_api.h>
#include <openbsc/bsc_rll.h>
#include <openbsc/gsm_data.h>
+#include <openbsc/gsm_subscriber.h>
#include <openbsc/signal.h>
#include <openbsc/abis_rsl.h>
#include <openbsc/chan_alloc.h>