build: fix Win32 clean build

The version.o file did not appear explicitly as a dependency, and
this caused clean builds to fail.  Force its build by making the
Makefile depend on version.o.

(We cannot add it to libqemuutil.a, because it doesn't export any
symbol and thus would not be pulled by the linker).

Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Stefan Weil <sw@weilnetz.de>
Tested-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>