gsmtap.h: Add definitions for ISDN PPP sub-type

This change adds a new sub-type for PPP in GSMTAP_TYPE_E1T1.

Change-Id: I17137167b2c74b6c5e53459743b6e140fc1a4386
diff --git a/include/osmocom/core/gsmtap.h b/include/osmocom/core/gsmtap.h
index 2387b20..ebb5296 100644
--- a/include/osmocom/core/gsmtap.h
+++ b/include/osmocom/core/gsmtap.h
@@ -185,6 +185,7 @@
 #define GSMTAP_E1T1_V120	0x08	/* V.120 B-channel data */
 #define GSMTAP_E1T1_V110	0x09	/* V.110 B-channel data */
 #define GSMTAP_E1T1_H221	0x0a	/* H.221 B-channel data */
+#define GSMTAP_E1T1_PPP		0x0b	/* PPP */
 
 /* flags for the ARFCN */
 #define GSMTAP_ARFCN_F_PCS	0x8000