selftests/bpf: Fix xdp_tx.c prog section name

The program type cannot be deduced from 'tx' which causes an invalid
argument error when trying to load xdp_tx.o using the skeleton.
Rename the section name to "xdp" so that libbpf can deduce the type.

Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210731055738.16820-7-joamaki@gmail.com