bpf, libbpf: Fix ___bpf_kretprobe_args1(x) macro definition

Use PT_REGS_RC instead of PT_REGS_RET to get ret correctly.

Fixes: df8ff353

 ("libbpf: Merge selftests' bpf_trace_helpers.h into libbpf's bpf_tracing.h")
Signed-off-by: Wenbo Zhang <ethercflow@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200315083252.22274-1-ethercflow@gmail.com