Fixing CXSparse include directories statement.

- Reported as issue #135:
  https://code.google.com/p/ceres-solver/issues/detail?id=135.
- CXSPARSE_INCLUDE was the legacy include directory variable, since
  the buildsystem updates we now use the CMake standard:
  CXSPARSE_INCLUDE_DIRS.

Change-Id: Iab0c2de14d524bb9e9da230bc574b5e6f09e1f31