Fix unused-function warning with Eigen < 3.2.2.

- CreateBlockJacobian() is only ever used when Eigen >= 3.2.2 is
  detected, but was previously defined whenever CERES_USE_EIGEN_SPARSE
  was defined with no check on the Eigen version.
- This resulted in an unused-function compile warning that became an
  error due to -Werror, preventing compilation when using Eigen < 3.2.2.

Change-Id: I24628ff329f14b087ece66bf2626bdc0de4ba224