tools/power turbostat: fix un-intended affinity of forked program

Linux 3.4 included a modification to turbostat to
lower cross-call overhead by using scheduler affinity:

15aaa346


(tools turbostat: reduce measurement overhead due to IPIs)

In the use-case where turbostat forks a child program,
that change had the un-intended side-effect of binding
the child to the last cpu in the system.

This change removed the binding before forking the child.

This is a back-port of a fix already included in turbostat v2.

Signed-off-by: Len Brown <len.brown@intel.com>