Generate version.h before adding subdirs (especially wsutil). Otherwise
build could fail in wsutil because of missing version.h

Change-Id: If6fdf1080c6de9ca1581bcc126506e78ec03a1d8
Reviewed-on: https://code.wireshark.org/review/2740
Reviewed-by: Jörg Mayer <jmayer@loplof.de>