Improve the performance of DenseQRSolver

1. Reduce amount of reallocations.

Change-Id: I91b17c781ae94ed12014d647f0162cfce4f6ed7b
diff --git a/internal/ceres/dense_qr_solver.h b/internal/ceres/dense_qr_solver.h
index dd683a8..f78fa72 100644
--- a/internal/ceres/dense_qr_solver.h
+++ b/internal/ceres/dense_qr_solver.h
@@ -33,6 +33,7 @@
 #define CERES_INTERNAL_DENSE_QR_SOLVER_H_
 
 #include "ceres/linear_solver.h"
+#include "ceres/internal/eigen.h"
 #include "ceres/internal/macros.h"
 
 namespace ceres {
@@ -90,6 +91,7 @@
       double* x);
 
   const LinearSolver::Options options_;
+  Vector rhs_;
   CERES_DISALLOW_COPY_AND_ASSIGN(DenseQRSolver);
 };