selftests/bpf: Remove unneeded semicolon

Eliminate the following coccicheck warning:
./tools/testing/selftests/bpf/test_flow_dissector.c:506:2-3: 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/1612780213-84583-1-git-send-email-yang.lee@linux.alibaba.com