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>