Change install location of kstyle config plugin

Currently the plugin is installed to /plasma/kcms/systemsettings_qwidgets, which is odd considering it's not a stanalone KCM that's supposed to be loaded by systemsettings directly

Instead it's loaded by the style KCM

Technically the exact location doesn't matter because the full path relative to the plugin dir is specified, so move it to a more appropriately named location