ipa: use %z as format string whne printing sizeof() result

thanks to Holger for reminding me
diff --git a/src/gsm/ipa.c b/src/gsm/ipa.c
index 9c66037..a49d8ac 100644
--- a/src/gsm/ipa.c
+++ b/src/gsm/ipa.c
@@ -355,7 +355,7 @@
 			}
 
 			LOGP(DLINP, LOGL_INFO,
-			     "Received part of IPA message header (%d/%lu)\n",
+			     "Received part of IPA message header (%d/%zu)\n",
 			     msg->len, sizeof(*hh));
 			if (!tmp_msg) {
 				ret = -EIO;