Move operator<< for FunctionSample back into ceres::internal.

This is undoing a change that Sergey submitted earlier moving
this into an anonymous namespace. This caused build breakage
internally at Google due to a namespace lookup error.

The original reason for Sergey's change was a lack of a
function declaration. I have added it before the function.

Change-Id: Iea88d1123215035e9f03022912078a2566ecde82