Fix compilation with Clang

Signed-off-by: Bartosz Brachaczek <b.brachaczek@gmail.com>
plugin: change return type of PluginManager::activatePluginWithDependencies

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: change return type of PluginManager::deactivatePluginWithDependents

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>