vm: Add missing module.

vm: Add missing module.

* gnu/build/vm.scm (define-module): Use module (guix build syscalls).

It fixes the following warnings during guix build :

gnu/build/vm.scm:233:3: warning: possibly unbound variable `mount'
gnu/build/vm.scm:238:3: warning: possibly unbound variable `umount'
gnu/build/vm.scm:268:8: warning: possibly unbound variable `mount'
gnu/build/vm.scm:276:8: warning: possibly unbound variable `umount'
gnu/build/vm.scm:315:4: warning: possibly unbound variable `mount'
gnu/build/vm.scm:323:4: warning: possibly unbound variable `umount'

This was not possible until the previous commit because we had to be sure
that Guile core implementation of 'mount' and 'umount' was used in
initrd context.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>