Fix a misleading cmake error message when KDecoration cannot be found
If KDecoration cannot be found, you would receive a very misleading error message such as
Must specify INSTALL_NAMESPACE for kcm_auroraedecoration
that doesn't provide any hints that KDecoration is missing. In order to address that, this change puts REQUIRED in find_package(), so cmake aborts the configure process earlier.
closes #182 (closed)
Edited by Vlad Zahorodnii