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: NAbaci Robot <abaci@linux.alibaba.com>
Signed-off-by: NYang Li <yang.lee@linux.alibaba.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1612780213-84583-1-git-send-email-yang.lee@linux.alibaba.com

tools/testing/selftests/bpf/test_flow_dissector.c

@@ -503,7 +503,7 @@ static int do_rx(int fd)
 		if (rbuf != cfg_payload_char)
 			error(1, 0, "recv: payload mismatch");
 		num++;
-	};
+	}
 
 	return num;
 }