initrd: Add the preprocessor guard in initrd.h

mainline inclusion
from mainline-v5.12-rc1-dontuse
commit fade5cad
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB


CVE: NA

--------------------------------

Add the preprocessor guard in initrd.h to prevent possible
build error from the multiple inclusion of same header file
multiple time.

Change-Id: I0644dec3e728545b6fb0fd451ffc454b3d09fc64
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>