Allow Common flags 0x94 and ignore them in PDP activation

extended from https://github.com/osmocom/openggsn/pull/2

Change-Id: I31b3e4b378e74bb5a0a2f54af6d2a15b629876cf
diff --git a/gtp/gtpie.h b/gtp/gtpie.h
index 754382e..8ea19ea 100644
--- a/gtp/gtpie.h
+++ b/gtp/gtpie.h
@@ -104,6 +104,7 @@
 #define GTPIE_EXT_HEADER_T  141	/* Extension Header Type List */
 #define GTPIE_TRIGGER_ID    142	/* Trigger Id */
 #define GTPIE_OMC_ID        143	/* OMC Identity */
+#define GTPIE_COMMON_FLAGS  148	/* Common Flags */
 #define GTPIE_RAT_TYPE      151	/* Radio Access Technology Type */
 #define GTPIE_USER_LOC      152	/* User Location Information  */
 #define GTPIE_MS_TZ         153	/* MS Time Zone */