Use public SIGRTMIN() and SIGRTMAX() from rust-vmm/vmm-sys-util

[iulianbarbu]

In #1434 we've added support for sigrtmin() and sigrtmax() (base signums for real-time signals) inside Firecracker. I've raised rust-vmm/vmm-sys-util#63 which asks for exporting these methods, which would ideally dissolve the support for these methods from within Firecracker. Raised this issue here for visibility.

[andreeaflorescu]

This is already merged and published in vmm-sys-util version 0.3.1. We just need to update the version and use them from there.