uefi: Fix compilation of minimal example

This turns out to be easier than I expected: just add `extern crate std;` so
that the panic handler and global allocator are set. The line is hidden so that
it doesn't appear in the rendered docs.

This allows the code block to `no_run` instead of `ignore`, so it will be
compiled (but not run, so there's no `main` linker error), and that also has the
nice side effect of removing the orange `!` warning bubble in the rendered docs.

Author: nicholasbishop

uefi/src/lib.rs

@@ -12,7 +12,7 @@
 //! Minimal example for an UEFI application using functionality of the
 //! `uefi` crate:
 //!
-//! ```ignore
+//! ```no_run
 //! #![no_main]
 //! #![no_std]
 //!
@@ -24,6 +24,7 @@
 //!
 //!     Status::SUCCESS
 //! }
+//! # extern crate std;
 //! ```
 //!
 //! Please find more info in our [Rust UEFI Book].