Add headset tag to getPass/submit;

api/lovr/headset/getPass.lua

@@ -1,4 +1,5 @@
 return {
+  tag = 'headset',
   summary = 'Get a Pass that renders to the headset.',
   description = 'Returns a `Pass` that renders to the headset display.',
   arguments = {},

api/lovr/headset/submit.lua

@@ -1,4 +1,5 @@
 return {
+  tag = 'headset',
   summary = 'Submit a frame to the headset display.',
   description = [[
     Submits the current headset texture to the VR display.  This should be called after calling