libkernel: remove leftover extern crate

It was the last one.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

rust/kernel/lib.rs

@@ -10,8 +10,6 @@
 #[cfg(not(CONFIG_RUST))]
 compile_error!("Missing kernel configuration for conditional compilation");
 
-extern crate alloc;
-
 use core::panic::PanicInfo;
 
 mod allocator;