[cmake] fix install directory for DuneUggridMacros.cmake

The install directory has to be ${DUNE_INSTALL_MODULEDIR}. Otherwise,
the file will not be found by dune_process_dependency_macros from
DuneMacros.cmake.

cmake/modules/CMakeLists.txt

-install(FILES DuneUggridMacros.cmake DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/cmake/modules)
+install(FILES DuneUggridMacros.cmake DESTINATION ${DUNE_INSTALL_MODULEDIR})