vmm: make default_syscalls module private
vmm: make default_syscalls module private

It had to be `pub` to be used by the demo jailer in the integration
tests, but that is no longer the case.

Signed-off-by: Alexandra Iordache <750ec4522de000f31ad44d64614ad5747f238032@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker