Update dummyguest to 2024 edition

Signed-off-by: Ludvig Liljenberg <lliljenberg@microsoft.com>

Author: ludfjig

src/tests/rust_guests/dummyguest/src/main.rs

@@ -42,7 +42,7 @@ fn mmio_read() {
 }
 
 #[allow(non_snake_case)]
-#[no_mangle]
+#[unsafe(no_mangle)]
 pub extern "win64" fn entrypoint(a: i64, b: i64, c: i32) -> i32 {
     if a != 0x230000 || b != 1234567890 || c != 4096 {
         mmio_read();