um: gitignore: Add kernel/capflags.c

This file is generated, we should ignore it.

Fixes: d8fb32f4 ("um: Add support for host CPU flags and alignment")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By:  <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>