kernel_tainted: use linux build tag instead of !windows (#847)

util.SysReadUintFromFile is defined in internal/util/sysreadfile.go
which has a //go:build linux constraint. Using !windows caused build
failures on OpenBSD and other non-Linux, non-Windows platforms.

/proc/sys/kernel/tainted is Linux-specific, so linux is the correct
build tag.

Signed-off-by: Arun Srinivasan <arun.srinivasan@flipkart.com>
Signed-off-by: Arun S <arun.srinivasan@flipkart.com>
Co-authored-by: Arun S <arun.srinivasan@flipkart.com>