Catch up to file renaming

This fixes "make install" which would otherwise fail with a diagnostic
message of the form

   CMake Error at cmake_install.cmake:64 (FILE):
     file INSTALL cannot find
     opm/autodiff/ConservativeSparseSparseProduct.h

Thanks to HudsonBuildServer for highlighting the issue.
Really catch up to file renaming.

Pointy hat: @bska
Merge pull request #262 from bska/fix-make-install

Fix "make install" in opm-autodiff