sync prototypes between the header and source

--- a/fec-pkt.c
+++ b/fec-pkt.c
@@ -68,3 +68,3 @@
 **/
-int fec_pkt_read(fec_pkt_t *pkt, int fd) {
+ssize_t fec_pkt_read(fec_pkt_t *pkt, int fd) {
   assert(pkt != NULL);