unstable-book: Add ignore to abi_efiapi example code

This example doesn't compile on targets that don't support UEFI, as
reported here:
rust-lang#104793 (comment)

Author: nicholasbishop

src/doc/unstable-book/src/language-features/abi-efiapi.md

@@ -12,7 +12,7 @@ Specification].
 
 Example:
 
-```rust
+```rust,ignore (not-all-targets-support-uefi)
 #![feature(abi_efiapi)]
 
 extern "efiapi" { fn f1(); }