MAINTAINERS: socket timestamping: add Jason Xing as reviewer

MAINTAINERS: socket timestamping: add Jason Xing as reviewer

Jason has been helping as reviewer for this area already, and has
contributed various features directly, notably BPF timestamping.

Also extend coverage to all timestamping tests, including those new
with BPF timestamping.

Link: https://lore.kernel.org/netdev/20250220072940.99994-1-kerneljasonxing@gmail.com/
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Link: https://patch.msgid.link/20250222172839.642079-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>