contrib/check-params-in-docs.py: Ignore gcn-preferred-vectorization-factor

contrib/check-params-in-docs.py: Ignore gcn-preferred-vectorization-factor

contrib/check-params-in-docs.py is a script that checks that all
options reported with ./gcc/xgcc -Bgcc --help=param are in
gcc/doc/invoke.texi and vice versa.
gcn-preferred-vectorization-factor is in the manual but normally not
reported by --help, probably because I do not have gcn offload
configured.  This patch makes the script silently about this particular
fact.

contrib/ChangeLog:

2024-04-11  Martin Jambor  <mjambor@suse.cz>

	* check-params-in-docs.py (ignored): Add
	gcn-preferred-vectorization-factor.