Fix warning C4373 in Visual Studio

The warning occurs because an overridden function added a const
to one argument.

Change-Id: Idd24f7c6ab60064747104bfc75ae9bf112f61b3e

internal/ceres/line_search.cc

diff --git a/internal/ceres/line_search.cc b/internal/ceres/line_search.cc
index 39618b5..8323896 100644
--- a/internal/ceres/line_search.cc
+++ b/internal/ceres/line_search.cc
@@ -112,7 +112,7 @@
   direction_ = direction;
 }
 
-bool LineSearchFunction::Evaluate(const double x, double* f, double* g) {
+bool LineSearchFunction::Evaluate(double x, double* f, double* g) {
   scaled_direction_ = x * direction_;
   if (!evaluator_->Plus(position_.data(),
                         scaled_direction_.data(),

internal/ceres/line_search.h

diff --git a/internal/ceres/line_search.h b/internal/ceres/line_search.h
index e4836b2..5f24e9f 100644
--- a/internal/ceres/line_search.h
+++ b/internal/ceres/line_search.h
@@ -231,7 +231,7 @@
   explicit LineSearchFunction(Evaluator* evaluator);
   virtual ~LineSearchFunction() {}
   void Init(const Vector& position, const Vector& direction);
-  virtual bool Evaluate(const double x, double* f, double* g);
+  virtual bool Evaluate(double x, double* f, double* g);
   double DirectionInfinityNorm() const;
 
  private: