gsmtap.h: Add definitions for various ISDN sub-types

We've started to encapsulate a varieth of ISDN related formats
as sub-types to GSMTAP_TYPE_E1T1 for quite some time.  Let's add
some more formats, such as V5EF, V.110, V.120, X.75 and H.221

For the HDLC based V5EF, V.110, V.120 and X.75 the payload format is
clear.  For V.110 and H.221 we will need to specify how the actual
payload looks like.

Change-Id: I8dee0f730cf91d6f9133d5c57f653e669ec8d598
diff --git a/include/osmocom/core/gsmtap.h b/include/osmocom/core/gsmtap.h
index 11b2d13..b0985f0 100644
--- a/include/osmocom/core/gsmtap.h
+++ b/include/osmocom/core/gsmtap.h
@@ -179,6 +179,11 @@
 #define GSMTAP_E1T1_RAW		0x03	/* raw/transparent B-channel */
 #define GSMTAP_E1T1_TRAU16	0x04	/* 16k TRAU frames; sub-slot 0-3 */
 #define GSMTAP_E1T1_TRAU8	0x05	/* 8k TRAU frames; sub-slot 0-7 */
+#define GSMTAP_E1T1_V5EF	0x06	/* V5 Envelope Function */
+#define GSMTAP_E1T1_X75		0x07	/* X.75 B-channel data */
+#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 */
 
 /* flags for the ARFCN */
 #define GSMTAP_ARFCN_F_PCS	0x8000