OvmfPkg/AcpiPlatformDxe: fix header file warts

OvmfPkg/AcpiPlatformDxe: fix header file warts

- Remove the leading underscores from the #include guard macro names;
  clean up the names in general.

- Remove the useless "Include/" directory prefix from the public header
  pathnames.

- Fix incorrect file-top comment.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210526201446.12554-7-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>