Fix CMake to execute the script src/svn_version.h (which generates the SVN support for Exiv2 -v -V).
Thanks to Mahesh for discovering this and letting me know.

git-svn-id: svn://dev.exiv2.org/svn@3278 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3