bpf_printk is a macro which is commonly used to print out debug messages
in BPF programs and it was copied in many selftests and samples. Since
all of them include bpf_helpers.h, this change moves the macro there.
Signed-off-by: NMichal Rostecki <mrostecki@opensuse.org>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>