selftests/bpf: Modify a test to check global functions

Make two static functions in test_xdp_noinline.c global:
before: processed 2790 insns
after: processed 2598 insns

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200110064124.1760511-6-ast@kernel.org