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/gtp/pdp.h b/gtp/pdp.h
index f8b0df8..106d544 100644
--- a/gtp/pdp.h
+++ b/gtp/pdp.h
@@ -17,6 +17,9 @@
 
 struct gsn_t;
 
+#define LOGPDPX(ss, level, pdp, fmt, args...)				\
+	LOGP(ss, level, "PDP(%s:%u): " fmt, imsi_gtp2str(&(pdp)->imsi), (pdp)->nsapi, ## args)
+
 #define PDP_MAX 1024		/* Max number of PDP contexts */
 #define PDP_MAXNSAPI 16		/* Max number of NSAPI */