gbproxy: Add BVC log filters

Since gbproxy doesn't use bssgp_rcvmsg from libosmocore we need to
implement our own filtering.

Change-Id: I4d1b57b89990945d307f27a58a7f630be0253d5b
Related: SYS#5232

include/osmocom/sgsn/gb_proxy.h

@@ -15,6 +15,9 @@
 
 #define GBPROXY_INIT_VU_GEN_TX 256
 
+/* BVCI uses 16 bits */
+#define BVC_LOG_CTX_FLAG (1<<17)
+
 struct rate_ctr_group;
 struct gprs_gb_parse_context;
 struct tlv_parsed;