[efi] Mark AppleNetBoot.h as a native iPXE header

AppleNetBoot.h is not taken from the EDK2 codebase and so cannot be
imported using include/ipxe/efi/import.pl.  Mark as a native iPXE
header (by changing the include guard) to avoid breaking the import
process.

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

src/include/ipxe/efi/Protocol/AppleNetBoot.h

-#ifndef __EFI_APPLE_NET_BOOT_PROTOCOL_H__
-#define __EFI_APPLE_NET_BOOT_PROTOCOL_H__
+#ifndef _IPXE_EFI_APPLE_NET_BOOT_PROTOCOL_H
+#define _IPXE_EFI_APPLE_NET_BOOT_PROTOCOL_H
 
 /** @file
  *
   GET_DHCP_RESPONSE	GetBsdpResponse;
 };
 
-#endif /*__EFI_APPLE_NET_BOOT_PROTOCOL_H__ */
+#endif /*_IPXE_EFI_APPLE_NET_BOOT_PROTOCOL_H */