wl_screenshooter: add missing XML protocol definition

modules/access/screen/screenshooter.xml

+<protocol name="screenshooter">
+
+  <interface name="screenshooter" version="1">
+    <request name="shoot">
+      <arg name="output" type="object" interface="wl_output"/>
+      <arg name="buffer" type="object" interface="wl_buffer"/>
+    </request>
+    <event name="done">
+    </event>
+  </interface>
+
+</protocol>