libgtp: Allow each PDP context to specify if it transmits G-PDU sequence numbers

GTP sequence numbers on GTP-U are optional for G-PDU type messages (i.e.
user-ip messages).  Let's allow the user to specify this behavior by
a new pdu_t.tx_gpdu_seq flag.  The flag is enabled by default to stay
compatible with the prior behaviour.

Related: OS#2519
Change-Id: Icf22a2ddd5c4a968ef5bda7c202b921d93fb49e6
diff --git a/TODO-RELEASE b/TODO-RELEASE
index d0852fc..e38e18c 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -7,3 +7,4 @@
 # If any interfaces have been added since the last public release: c:r:a + 1.
 # If any interfaces have been removed or changed since the last public release: c:r:0.
 #library	what		description / commit summary line
+libgtp		pdp.h		Addition of new tx_gpdu_seq struct member member