llc: Count timedout and silently dropped frames

A DL tbf can be discarded and then the already queued LLCs will
be silently dropped. Count this event.
diff --git a/src/bts.cpp b/src/bts.cpp
index 3f9b24a..07e97de 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -57,6 +57,8 @@
 	{ "sba.allocated",		"SBA Allocated        "},
 	{ "sba.freed",			"SBA Freed            "},
 	{ "sba.timedout",		"SBA Timeout          "},
+	{ "llc.timeout",		"Timedout Frames      "},
+	{ "llc.dropped",		"Dropped Frames       "},
 };
 
 static const struct rate_ctr_group_desc bts_ctrg_desc = {