[romprefix] Add missing FILE_LICENCE declaration to undiloader.S

undiloader.S was originally part of romprefix.S, and so inherits its
licence.

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

src/arch/i386/prefix/undiloader.S

+FILE_LICENCE ( GPL2_OR_LATER )
+
 	.text
 	.code16
 	.arch i386