libbpf: Use memcpy instead of strncpy to please GCC

Some versions of GCC are really nit-picky about strncpy() use. Use memcpy(),
as they are pretty much equivalent for the case of fixed length strings.

Fixes: e459f49b ("libbpf: Separate XDP program load with xsk socket creation")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201203235440.2302137-1-andrii@kernel.org