fix compiler warnings (itsme <itsme@xs4all.nl>)

 * added several 'const' for strings.
 * added 'extern' to declarations of rsl_rlm_cause_strs
diff --git a/src/gsm_utils.c b/src/gsm_utils.c
index 97497c8..593dd5c 100644
--- a/src/gsm_utils.c
+++ b/src/gsm_utils.c
@@ -193,7 +193,7 @@
 	return rxlev;
 }
 
-char *gsm_band_name(enum gsm_band band)
+const char *gsm_band_name(enum gsm_band band)
 {
 	switch (band) {
 	case GSM_BAND_450: