[autoconf,bugfix] Fixed typo in name of CMake macro.

DuneCmakePackageConfigHelpers.cmake -> DuneCMakePackageConfigHelpers.cmake
(cherry picked from commit 90ebe8970f5d9687f2f9915cb8b8e65153e1d65a)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>