[build] Mark core files as permitted for UEFI Secure Boot

Mark all files used in a standard build of bin-x86_64-efi/snponly.efi
as permitted for UEFI Secure Boot.  These files represent the core
functionality of iPXE that is guaranteed to have been included in
every binary that was previously subject to a security review and
signed by Microsoft.  It is therefore legitimate to assume that at
least these files have already been reviewed to the required standard
multiple times.

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

src/include/stdarg.h

@@ -2,6 +2,7 @@
 #define _STDARG_H
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+FILE_SECBOOT ( PERMITTED );
 
 typedef __builtin_va_list va_list;
 #define va_start( ap, last ) __builtin_va_start ( ap, last )