Fix type of suppressed compiler warning for Eigen 3.2.0.

- Warning type generated is unused-but-set-variable, not unused-variable
  and the corresponding "local variable is initialized but not
  referenced" MSVC warning.

Change-Id: I716fd6a87cc91533bc862b2ef932601c8857818d

internal/ceres/covariance_impl.cc

diff --git a/internal/ceres/covariance_impl.cc b/internal/ceres/covariance_impl.cc
index 0a536f8..821be49 100644
--- a/internal/ceres/covariance_impl.cc
+++ b/internal/ceres/covariance_impl.cc
@@ -45,10 +45,10 @@
 // widely used (Ubuntu 14.04), and Ceres won't compile otherwise due to -Werror.
 #if defined(_MSC_VER)
 #pragma warning( push )
-#pragma warning( disable : 42024 )
+#pragma warning( disable : 4189 )
 #else
 #pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunused-variable"
+#pragma GCC diagnostic ignored "-Wunused-but-set-variable"
 #endif
 #include "Eigen/SparseQR"
 #if defined(_MSC_VER)