libbpf: print error to error stream
libbpf: print error to error stream

making it consistent with rest of the code

mcaleavya/bcc,brendangregg/bcc,mcaleavya/bcc,iovisor/bcc,brendangregg/bcc,mcaleavya/bcc,tuxology/bcc,mcaleavya/bcc,iovisor/bcc,mcaleavya/bcc,iovisor/bcc,tuxology/bcc,brendangregg/bcc,iovisor/bcc,tuxology/bcc,iovisor/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc