Move extended PDP logging macro to header

It might be useful for any user of libgtp who uses libosmocore so let's
make generalized version of it available as part of installable header.

Change-Id: I79aba10ef989384a28f059c30899e65c771ae5e1
Related: SYS#3610
diff --git a/TODO-RELEASE b/TODO-RELEASE
index e38e18c..1009c44 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -8,3 +8,4 @@
 # 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
+libgtp		pdp.h		add LOGPDPX() helper to public API