Add missing core::panic::PanicInfo imports in code snippets (#440)

blog/content/second-edition/posts/01-freestanding-rust-binary/index.md

@@ -200,6 +200,8 @@ To tell the Rust compiler that we don't want to use the normal entry point chain
 #![no_std]
 #![no_main]
 
+use core::panic::PanicInfo;
+
 /// This function is called on panic.
 #[panic_implementation]
 #[no_mangle]
@@ -313,6 +315,8 @@ A minimal freestanding Rust binary looks like this:
 #![no_std] // don't link the Rust standard library
 #![no_main] // disable all Rust-level entry points
 
+use core::panic::PanicInfo;
+
 /// This function is called on panic.
 #[panic_implementation]
 #[no_mangle]