ceres::internal::FixedArray -> absl::FixedArray

Ceres Solver was using an old forked version of FixedArray,
now that we are using absl, we can use the official version
that ships with it.

Change-Id: Ic88d7f6e8a49b928d611f7cbb04172452b322b01

include/ceres/numeric_diff_cost_function.h

@@ -228,7 +228,7 @@ class NumericDiffCostFunction final
   bool Evaluate(double const* const* parameters,
                 double* residuals,
                 double** jacobians) const override {
-    using internal::FixedArray;
+    using absl::FixedArray;
     using internal::NumericDiff;
 
     using ParameterDims =