GTP: use socklen_t where appropriate

this fixes some compiler warnings
diff --git a/gtp/gtp.c b/gtp/gtp.c
index 768d8c7..dc3a05d 100644
--- a/gtp/gtp.c
+++ b/gtp/gtp.c
@@ -2718,7 +2718,7 @@
 {
 	unsigned char buffer[PACKET_MAX];
 	struct sockaddr_in peer;
-	size_t peerlen;
+	socklen_t peerlen;
 	int status;
 	struct gtp0_header *pheader;
 	int version = 0;	/* GTP version should be determined from header! */
@@ -2863,7 +2863,7 @@
 {
 	unsigned char buffer[PACKET_MAX];
 	struct sockaddr_in peer;
-	size_t peerlen;
+	socklen_t peerlen;
 	int status;
 	struct gtp1_header_short *pheader;
 	int version = 1;	/* TODO GTP version should be determined from header! */
@@ -3038,7 +3038,7 @@
 {
 	unsigned char buffer[PACKET_MAX];
 	struct sockaddr_in peer;
-	size_t peerlen;
+	socklen_t peerlen;
 	int status;
 	struct gtp1_header_short *pheader;
 	int version = 1;	/* GTP version should be determined from header! */