build system: Do always regenerate version.o

We make two changes which affect only the litle recursive make of
version.o.  We only build version.o in the little sub-make when
relinking and then we want to always do it.  So the setting of OBJECTS
is pointless, and instead we make version.c depend on FORCE.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>