template: fix build failure

[JeffLi01]

The template failed to build when built with cmds below:

cd template
cargo build --target x86_64-unknown-uefi

We'll get error:

error: no global memory allocator found but one is required; link to std or add `#[global_allocator]` to a static item that implements the GlobalAlloc trait

error: `#[panic_handler]` function required, but not found

error: could not compile `uefi_app` (bin "uefi_app") due to 2 previous errors

As the Rust UEFI Book shown, the template succeeds to build with uefi-rs feature panic_handler enabled.

[phip1611]

Yikes, sorry that you ran into that. The proper solution is #1170

ping @nicholasbishop

[nicholasbishop]

Thanks for the fix. I'll go ahead and merge, we can revisit after resolving 1170.