dos2unix: fix static build

dos2unix: fix static build

Pass the -static flag to the final link command. Otherwise, the
generated executables are not static when the external toolchain
provides both shared and static libraries.

Should also fix:
http://autobuild.buildroot.net/results/a5a/a5a64bc3ff5a91a8680ac52f3505362491923b26/
http://autobuild.buildroot.net/results/ebf/ebfe81e65751d7a5ca3aa4e20cc708a24ba65204/

Cc: David Bachelart <david.bachelart@bbright.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>