pixman: ignore CVE-2023-37769

This issue relates to a floating point exception in stress-test, which
is an unlikely security exploit at the best of times, but the test is
not installed so isn't relevant.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit a36d62a0)
Signed-off-by: Steve Sakoman <steve@sakoman.com>