Export names for gsm_chan_t and gsm48_chan_mode

This can be used with get_value_string() to improve debugging output.
diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index 6458447..99aae65 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -28,6 +28,7 @@
 #include <stdint.h>
 
 #include <osmocom/core/defs.h>
+#include <osmocom/core/utils.h>
 
 #define ADD_MODULO(sum, delta, modulo) do {	\
 	if ((sum += delta) >= modulo)		\
@@ -199,6 +200,8 @@
 	_GSM_LCHAN_MAX
 };
 
+extern const struct value_string gsm_chan_t_names[];
+
 /* Deprectated functions */
 /* Limit encoding and decoding to use no more than this amount of buffer bytes */
 #define GSM_7BIT_LEGACY_MAX_BUFFER_SIZE  0x10000