loader: Make arm64_image_header public

This header is equivalent to the boot_params header found at the top of x86 kernels. We want to make this struct public to use it in a fw_cfg implementation that reads the kernel header for both arm and x86

Signed-off-by: Alex Orozco <alexorozco@google.com>

Author: AlexOrozco1256

src/loader/pe/mod.rs

@@ -75,7 +75,7 @@ impl std::error::Error for Error {}
 #[derive(Debug, Copy, Clone, Default)]
 // See kernel doc Documentation/arm64/booting.txt for more information.
 // All these fields should be little endian.
-struct arm64_image_header {
+pub struct arm64_image_header {
     code0: u32,
     code1: u32,
     text_offset: u64,