FreeBSD: Ignore symlink to i386 includes

A symlink to i386 includes is created in the build dir on amd64 since
freebsd/freebsd-src@d07600c563039f252becc29ac7d9a454b6b0600d

Tell git to ignore it like the other include links.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #13719