Gb: Add header file for BSS-side BSSGP functions

.. not sure how we could have missed that so far.
diff --git a/src/gb/gprs_bssgp_bss.c b/src/gb/gprs_bssgp_bss.c
index 9d0325a..24b0b18 100644
--- a/src/gb/gprs_bssgp_bss.c
+++ b/src/gb/gprs_bssgp_bss.c
@@ -29,6 +29,7 @@
 #include <osmocom/gsm/tlv.h>
 #include <osmocom/core/talloc.h>
 #include <osmocom/gprs/gprs_bssgp.h>
+#include <osmocom/gprs/gprs_bssgp_bss.h>
 #include <osmocom/gprs/gprs_ns.h>
 
 #include "common_vty.h"