selftests/bpf: Fix checkpatch error on empty function parameter

Fix checkpatch error: "ERROR: Bad function definition - void foo()
should probably be void foo(void)". Most replacements are done by
the following command:

  sed -i 's#\([a-z]\)()$#\1(void)#g' testing/selftests/bpf/benchs/*.c

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211210141652.877186-3-houtao1@huawei.com