use _NUM_CHREQ_T to define the size of ctype_by_chreq

Make it future compatible for changes. Otherwise it will break
when additional enums are adding to chreq in libosmocore.
Depends on libosmocore Id67ba8de89dd6288e449197438e9e1c5d7f5a134

Change-Id: I2acab2af8d67bccb2bc495512c1f259ae649a832
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 91aee43..1ca3bc9 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -395,7 +395,7 @@
 		enum rrlp_mode mode;
 	} rrlp;
 
-	enum gsm_chan_t ctype_by_chreq[18];
+	enum gsm_chan_t ctype_by_chreq[_NUM_CHREQ_T];
 
 	/* Use a TCH for handling requests of type paging any */
 	int pag_any_tch;