Correct prefix path.

[[Imported from SVN: r1720]]

cmake/pkg/dune-istl-config.cmake.in

 if(NOT @DUNE_MOD_NAME@_FOUND)
 #compute installation prefix relative to this file
 get_filename_component(_dir "${CMAKE_CURRENT_LIST_FILE}" PATH)
-get_filename_component(_prefix "${_dir}/../.." ABSOLUTE)
+get_filename_component(_prefix "${_dir}/../../.." ABSOLUTE)
 
 #import the target
 #include("${_prefix}/lib/cmake/@DUNE_MOD_NAME@-targets.cmake")