selftests/bpf: Fix compiler warning in BPF_KPROBE definition in loop6.c

Add missing return type to BPF_KPROBE definition. Without it, compiler
generates the following warning:

progs/loop6.c:68:12: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
BPF_KPROBE(trace_virtqueue_add_sgs, void *unused, struct scatterlist **sgs,
           ^
1 warning generated.

Fixes: 86a35af6 ("selftests/bpf: Add a verifier scale test with unknown bounded loop")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210309044322.3487636-1-andrii@kernel.org