Summary:
Fixes a CMake warning:
"Your project already requires a version of CMake that includes the
find_dependency macro via the CMakeFindDependencyMacro module.  You
should use CMakePackageConfigHelpers instead of
ECMPackageConfigHelpers"

None of the macros from ECMPackageConfigHelpers is actually used.

Test Plan: Builds, warning disappears

Reviewers: #digikam

Differential Revision: https://phabricator.kde.org/D7728