webos_enhanced_submissions: Add P_V_${PN} to PV vardeps

* without this, changes to PREFERRED_VERSION_foo do not change
  sstate checksums of foo tasks and recipes which depend on foo
  are not rebuilt

[OWEBOS-3112] Changing PREFERRED_VERSION does not invalidate sstate
              checksums of depending components

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@palm.com>
Change-Id: I0124c71e13dca674d0d64ba7f62073a974593522
Reviewed-on: http://g2g.palm.com/1218
Reviewed-by: Martin Jansa <martin.jansa@palm.com>
Tested-by: Martin Jansa <martin.jansa@palm.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@palm.com>