Suppress unused variable warning from Eigen 3.2.0.

- Eigen 3.2.0 emits an unused variable warning from Ordering.h
  #included by Eigen/SparseQR which prevents Ceres compilation due
  to -Werror.
- As Eigen 3.2.0 is still in widespread use, we explicitly disable
  the warning only around the inclusion of Eigen/SparseQR.

Change-Id: I48ce905ef6a22bc1b06f9e5bed328bd315f407f0