LoongArch: Move struct kimage forward declaration before use

arch_kimage_file_post_load_cleanup() and load_other_segments(), both
inside the CONFIG_KEXEC_FILE block, take a struct kimage pointer before
the forward declaration appears. Move the forward declaration above so
it precedes its first use instead of relying on a transitive include.

Fixes: d162feec6b6e ("LoongArch: Add preparatory infrastructure for kexec_file")
Signed-off-by: 谢致邦 (XIE Zhibang) <Yeking@Red54.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>