GSMTAP: add definitions for LTE NAS subtypes

Wireshark GSMTAP dissector has LTE NAS subtypes for plain NAS messages
and NAS messages with security header. This commit adds definition for
these subtypes.

Change-Id: I49d15fb13299c1bcbe0fb7e50bf222294c7a49ef
diff --git a/include/osmocom/core/gsmtap.h b/include/osmocom/core/gsmtap.h
index 9e3db0b..9c2965b 100644
--- a/include/osmocom/core/gsmtap.h
+++ b/include/osmocom/core/gsmtap.h
@@ -272,6 +272,14 @@
 	GSMTAP_LTE_RRC_SUB_MAX
 };
 
+/* LTE NAS message types */
+enum {
+	GSMTAP_LTE_NAS_PLAIN = 0,
+	GSMTAP_LTE_NAS_SEC_HEADER,
+
+	GSMTAP_LTE_NAS_SUB_MAX
+};
+
 /* ====== DO NOT MAKE UNAPPROVED MODIFICATIONS HERE ===== */
 /*! Structure of the GTMTAP pseudo-header */
 struct gsmtap_hdr {