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

internal/ceres/line_search.cc

@@ -67,15 +67,17 @@ FunctionSample ValueAndGradientSample(const double x,
   return sample;
 };
 
+}  // namespace
+
+
+std::ostream& operator<<(std::ostream &os, const FunctionSample& sample);
+
 // Convenience stream operator for pushing FunctionSamples into log messages.
-std::ostream& operator<<(std::ostream &os,
-                         const FunctionSample& sample) {
+std::ostream& operator<<(std::ostream &os, const FunctionSample& sample) {
   os << sample.ToDebugString();
   return os;
 }
 
-}  // namespace
-
 LineSearch::LineSearch(const LineSearch::Options& options)
     : options_(options) {}