net: lib: http_server: fix snprintk issue of size_t

The size_t type may vary from machines. Current snprintk code was
causing below build issue on arm64.

error: format '%x' expects argument of type 'unsigned int', but argument
4 has type 'size_t' {aka 'long unsigned int'} [-Werror=format=].

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>

subsys/net/lib/http/http_server_http1.c

@@ -237,7 +237,7 @@ static int http1_dynamic_response(struct http_client_ctx *client, struct http_re
 
  /* Send body data if provided */
  if (rsp->body != NULL && rsp->body_len > 0) {
- ret = snprintk(tmp, sizeof(tmp), "%x\r\n", rsp->body_len);
+ ret = snprintk(tmp, sizeof(tmp), "%zx\r\n", rsp->body_len);
  ret = http_server_sendall(client, tmp, ret);
  if (ret < 0) {
  return ret;