gsm: gsm_gsmtime2fn(): constify param

Change-Id: Ib29d5a8e7dcf50fb086f967857b0afa076573163
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index 48341ae..06b2406 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -885,7 +885,7 @@
 /*! Encode decoded \ref gsm_time to Frame Number
  *  \param[in] time GSM Time in decoded structure
  *  \returns GSM Frame Number */
-uint32_t gsm_gsmtime2fn(struct gsm_time *time)
+uint32_t gsm_gsmtime2fn(const struct gsm_time *time)
 {
 	uint32_t fn;