comps: install qubes-usb-proxy-dom0 by default

This package isn't strictly required for qvm-usb to work, but contains
default ("deny all") policy to mute qrexec prompt. And tests.

conf/comps-qubes.xml

@@ -1165,6 +1165,7 @@
       <packagereq type="default">qubes-gpg-split-dom0</packagereq>
       <packagereq type="default">qubes-pdf-converter-dom0</packagereq>
       <packagereq type="default">qubes-input-proxy</packagereq>
+      <packagereq type="default">qubes-usb-proxy-dom0</packagereq>
     </packagelist>
   </group>