gbproxy: Add config option to name an SGSN

This is useful for logging and configuration to identify an SGSN by name

Change-Id: I2a3410dd9bebb242957e13a63ed70e447204203c
Related: SYS#5115, OS#4472
diff --git a/src/gbproxy/gb_proxy.c b/src/gbproxy/gb_proxy.c
index d86c6ae..34cff31 100644
--- a/src/gbproxy/gb_proxy.c
+++ b/src/gbproxy/gb_proxy.c
@@ -302,7 +302,7 @@
 	}
 
 	/* This shouldn't happen */
-	LOGPCELL(cell, LOGL_ERROR, "Could not find matching BVC for SGSN, dropping message!\n");
+	LOGPCELL(cell, LOGL_ERROR, "Could not find matching BVC for SGSN %s, dropping message!\n", sgsn->name);
 	return NULL;
 }