[build] Mark dummy architecture headers as permitted for UEFI Secure Boot

The dummy header files in include/bits/*.h are placeholders for
architectures that do not need to define any architecture-specific
functionality in these areas.  Mark these trivial files as permitted
for UEFI Secure Boot.

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

src/include/bits/hyperv.h

@@ -11,5 +11,6 @@
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+FILE_SECBOOT ( PERMITTED );
 
 #endif /* _BITS_HYPERV_H */