Import serial_println only in test mode

src/vga_buffer.rs

@@ -1,9 +1,11 @@
-use crate::{serial_print, serial_println};
 use core::fmt;
 use lazy_static::lazy_static;
 use spin::Mutex;
 use volatile::Volatile;
 
+#[cfg(test)]
+use crate::{serial_print, serial_println};
+
 lazy_static! {
     /// A global `Writer` instance that can be used for printing to the VGA text buffer.
     ///