Add ability get kernel version from header at runtime
Add ability get kernel version from header at runtime

Useful if bpftrace is compiled on a different kernel version
than it's being run on.

iovisor/bpftrace,ajor/bpftrace,ajor/bpftrace,iovisor/bpftrace,ajor/bpftrace,iovisor/bpftrace,iovisor/bpftrace