fix: add plugin attribute to webservices manifest files element

Joomla's PluginAdapter requires plugin="xxx" on a <files> child to
determine filesystem deployment path and extension element. Without it,
plugin files are not deployed and the element field is empty in
#__extensions. Pattern confirmed from MokoSuiteClient.

Authored-by: Moko Consulting

source/packages/plg_webservices_mokosuitestorelocator/plg_webservices_mokosuitestorelocator.xml

@@ -17,7 +17,7 @@
 	<namespace path="src">Moko\Plugin\WebServices\MokoSuiteStoreLocator</namespace>
 
 	<files>
-		<folder>services</folder>
+		<folder plugin="mokosuitestorelocator">services</folder>
 		<folder>src</folder>
 		<folder>language</folder>
 	</files>