Only warn about missing BTF info in debug mode
Only warn about missing BTF info in debug mode

Before:

    $ sudo ./build/src/bpftrace -e 'BEGIN { printf("hello world\n"); }'
    BTF: failed to find BTF data
    Attaching 1 probe...
    hello world
    ^C

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