bpf/benchs/bench_ringbufs: Remove unneeded semicolon

Eliminate the following coccicheck warning:
./tools/testing/selftests/bpf/benchs/bench_ringbufs.c:322:2-3: Unneeded
semicolon

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1612684360-115910-1-git-send-email-yang.lee@linux.alibaba.com