virtualbox6: add 'Audio_in' session requirement to pkg

To actually receive audio input in a particular VM, it also
needs to be enabled in the vbox file:

<AudioAdapter controller="HDA" driver="OSS" enabled="true"
              enabledOut="true" enabledIn="true"/>

Fixes #4377

repos/ports/recipes/pkg/vbox6/runtime

@@ -17,6 +17,7 @@
 		<rom label="usb_devices"/>
 		<usb/>
 		<audio_out/>
+		<audio_in/>
 	</requires>
 
 	<config verbose="yes">