[efi] Standardise #include guard in ipxe_download.h

The script include/ipxe/efi/import.pl relies on a particular format
for the #include guard in order to detect EFI headers that are not
imported.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/efi/ipxe_download.h

+#ifndef _IPXE_DOWNLOAD_H
+#define _IPXE_DOWNLOAD_H
+
 /*
  * Copyright (C) 2010 VMware, Inc.  All Rights Reserved.
  *
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#ifndef IPXE_DOWNLOAD_H
-#define IPXE_DOWNLOAD_H
-
 FILE_LICENCE ( GPL2_OR_LATER );
 
 /** @file
     0x3eaeaebd, 0xdecf, 0x493b, { 0x9b, 0xd1, 0xcd, 0xb2, 0xde, 0xca, 0xe7, 0x19 } \
   }
 
-#endif
+#endif /* _IPXE_DOWNLOAD_H */