As reported in bug 401888, the current kcoreaddons_add_plugin CMake macro causes deprecation warnings in extra-cmake-module's KDEInstallDirs since we use an old-style install path variable (PLUGIN_INSTALL_DIR).

This should just be KDE_INSTALL_PLUGINDIR and so that's all my fix is, but since I'm not very familiar with the CMake stuff or the plugin loading code I wanted to get a review on it before I commit.

Built and installed kcoreaddons successfully, and then built and installed kactivitymanagerd (which uses kcoreaddons_add_plugin pervasively) and verified that kactivitymanagerd still builds and installs and that the plugin install dir, including its install namespace, is properly updated.

I also manually forced KDE_INSTALL_DIRS_NO_DEPRECATED to a true value in kactivitymanagerd as recommended in the bug, and still rebuilt/installed without deprecation warnings.