collect_gpl_sources.mk: Declare rule to assemble GPL sources FORCE

We have no rigorous way to detect whether the code referenced for this
mechanism changes in between builds. So declare this as a phony target
so that it is rebuilt every time.

Change-Id: Ie1670d96895719d406e7b0defd53ba3cd224dd83
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>