selftests: bpf: Move bpf_printk to bpf_helpers.h 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: Michal Rostecki <[email protected]> Signed-off-by: Alexei Starovoitov <[email protected]>