gtp-genl: fix compilation warning

Fix these compilation warning:

gtp-genl.c:131:8: warning: 'pdp.version' may be used uninitialized in this function [-Wmaybe-uninitialized]
gtp-genl.c:132:8: warning: 'pdp.tid' may be used uninitialized in this function [-Wmaybe-uninitialized]
d  CCLD   libgtpnl.la

By initializing the pdp object.
diff --git a/src/gtp-genl.c b/src/gtp-genl.c
index 4891b08..43ffb30 100644
--- a/src/gtp-genl.c
+++ b/src/gtp-genl.c
@@ -111,7 +111,7 @@
 static int genl_gtp_attr_cb(const struct nlmsghdr *nlh, void *data)
 {
 	struct nlattr *tb[GTPA_MAX + 1] = {};
-	struct gtp_pdp pdp;
+	struct gtp_pdp pdp = {};
 	struct genlmsghdr *genl;
 
 	mnl_attr_parse(nlh, sizeof(*genl), genl_gtp_validate_cb, tb);