[crypto] Remove obsolete extern declaration for asn1_invalidate_cursor()

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

src/include/ipxe/asn1.h

 	return ( ( cursor->len >= sizeof ( *type ) ) ? *type : ASN1_END );
 }
 
-extern void asn1_invalidate_cursor ( struct asn1_cursor *cursor );
 extern int asn1_enter ( struct asn1_cursor *cursor, unsigned int type );
 extern int asn1_skip_if_exists ( struct asn1_cursor *cursor,
 				 unsigned int type );