stats: Attempt to compile fix the new rate_ctr

We wanted to support gcc-4.2 and this didn't allow us to use
the C99 initializers inside C++ code. Attempt to initialize
the class_id correctly.
diff --git a/src/bts.cpp b/src/bts.cpp
index 122ff20..78d1de5 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -80,6 +80,7 @@
 static const struct rate_ctr_group_desc bts_ctrg_desc = {
 	"bts",
 	"BTS Statistics",
+	OSMO_STATS_CLASS_GLOBAL,
 	ARRAY_SIZE(bts_ctr_description),
 	bts_ctr_description,
 };