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

tools/lib/bpf/xsk.c

@@ -583,7 +583,7 @@ static int xsk_create_xsk_struct(int ifindex, struct xsk_socket *xsk)
 	}
 
 	ctx->ifindex = ifindex;
-	strncpy(ctx->ifname, ifname, IFNAMSIZ - 1);
+	memcpy(ctx->ifname, ifname, IFNAMSIZ -1);
 	ctx->ifname[IFNAMSIZ - 1] = 0;
 
 	xsk->ctx = ctx;