Fix missing ABI warnings for Rust 1.86

[randomnoise]

When running cargo build --package plato-core with Rust 1.86.0+, warnings are printed similar to this:

warning: extern declarations without an explicit ABI are deprecated
  --> crates/core/src/document/mupdf_sys.rs:45:1
   |
45 | extern {
   | ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`

This PR makes explicit extern C ABI declarations and fixes these warnings.

For more info:

• https://blog.rust-lang.org/2025/04/03/Rust-1.86.0/#make-missing-abi-lint-warn-by-default
• https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#missing-abi

[aykevl]

I think you missed one: crates/core/src/font/mod.rs:103
The warning is only shown when running ./build.sh, not when running the emulator.