gprs_ns2: Introduce gprs_ns2_lltype_str() for link layer name

Change-Id: I4b257c09d9d3c97d6da04240c7a74a95c7c93675
diff --git a/src/gb/gprs_ns2.c b/src/gb/gprs_ns2.c
index 470027a..30716fc 100644
--- a/src/gb/gprs_ns2.c
+++ b/src/gb/gprs_ns2.c
@@ -211,6 +211,13 @@
 	{ 0, NULL }
 };
 
+const struct value_string gprs_ns2_lltype_strs[] = {
+	{ GPRS_NS2_LL_UDP,	"UDP" },
+	{ GPRS_NS2_LL_FR_GRE,	"FR_GRE" },
+	{ GPRS_NS2_LL_FR,	"FR" },
+	{ 0, NULL }
+};
+
 /*! string-format a given NS-VC into a user-supplied buffer.
  *  \param[in] buf user-allocated output buffer
  *  \param[in] buf_len size of user-allocated output buffer in bytes