Add strings for GPRS ciphers

Change-Id: I050bda5fb50aad42fe2d72de39f8256b4fb12fb2
Related: OS#1582
diff --git a/include/osmocom/crypt/gprs_cipher.h b/include/osmocom/crypt/gprs_cipher.h
index d6edefa..9d98f88 100644
--- a/include/osmocom/crypt/gprs_cipher.h
+++ b/include/osmocom/crypt/gprs_cipher.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <osmocom/core/linuxlist.h>
+#include <osmocom/core/utils.h>
 
 #define GSM0464_CIPH_MAX_BLOCK	1523
 
@@ -18,6 +19,8 @@
 	GPRS_CIPH_SGSN2MS,
 };
 
+extern const struct value_string gprs_cipher_names[];
+
 /* An implementation of a GPRS cipher */
 struct gprs_cipher_impl {
 	struct llist_head list;