use ssize_t for return value

Co-authored-by: Etienne Millon <etienne.millon@gmail.com>
Signed-off-by: Haoxiang Fei <tonyfettes@tonyfettes.com>

otherlibs/stdune/src/copyfile_stubs.c

@@ -74,7 +74,7 @@ CAMLprim value stdune_copyfile(value v_from, value v_to) {
 }
 
 static int dune_sendfile(int in, int out, size_t length) {
-  int ret;
+  ssize_t ret;
   while (length > 0) {
     ret = sendfile(out, in, NULL, length);
     if (ret < 0) {