ArmPkg: Rename include guard in ArmGicLib.h

ArmPkg: Rename include guard in ArmGicLib.h

Edk2 coding standard states that:
"Names starting with one or two underscores, such as
_MACRO_GUARD_FILE_NAME_H_, must not be used."

Ref:
https://edk2-docs.gitbook.io/edk-ii-c-coding-standards-specification/
5_source_files/53_include_files#
5-3-5-all-include-file-contents-must-be-protected-by-a-include-guard

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>