libosmogb: export btsctx_* functions

Those should be private, but osmo-pcu currently needs them...
diff --git a/src/gb/libosmogb.map b/src/gb/libosmogb.map
index 81b04c4..a41a91a 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -57,5 +57,9 @@
 
 gprs_log_filter_fn;
 
+btsctx_alloc;
+btsctx_by_bvci_nsei;
+btsctx_by_raid_cid;
+
 local: *;
 };