.travis.yml: default the --disable-system build to --static

It's fairly common to build qemu-user binaries with --static linking
so the binary can be copied around without libraries. Enable --static
in the default qemu-user build to cover this.

There are other qemu-user builds that use dynamic linking so they
should catch any problems there.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>