Fix private macro errors on nightly rust toolchains

When compiling the rust template using the nightly toolchain, the following error occurs:

error: macro `cpp` is private
  --> src/main.rs:29:9
   |
29 |         cpp! { {
   |         ^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #120192 <https://github.com/rust-lang/rust/issues/120192>
   = note: `#[deny(private_macro_use)]` on by default

Importing cpp::cpp explicitly fixes this.