Fix size_of import

Author: kornelski

src/codecs/pnm/decoder.rs

@@ -1,6 +1,7 @@
 use std::error;
 use std::fmt::{self, Display};
 use std::io::{self, Read};
+use std::mem::size_of;
 use std::num::ParseIntError;
 use std::str::{self, FromStr};
 

src/image.rs

@@ -1,6 +1,7 @@
 #![allow(clippy::too_many_arguments)]
 use std::ffi::OsStr;
 use std::io::{self, Write};
+use std::mem::size_of;
 use std::ops::{Deref, DerefMut};
 use std::path::Path;