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>

ArmPkg/Include/Library/ArmGicLib.h

@@ -1,13 +1,13 @@
 /** @file
 *
-*  Copyright (c) 2011-2018, ARM Limited. All rights reserved.
+*  Copyright (c) 2011-2021, Arm Limited. All rights reserved.<BR>
 *
 *  SPDX-License-Identifier: BSD-2-Clause-Patent
 *
 **/
 
-#ifndef __ARMGIC_H
-#define __ARMGIC_H
+#ifndef ARMGIC_H_
+#define ARMGIC_H_
 
 #include <Library/ArmGicArchLib.h>
 
@@ -333,4 +333,4 @@ ArmGicV3SetPriorityMask (
   IN UINTN                  Priority
   );
 
-#endif
+#endif // ARMGIC_H_