MINOR: [C++] Fix MSVC compiler error

[pitrou]

Downcast explicitly from double to long.

This fixes a regression introduced in ARROW-16521.

[pitrou]

CI passed on my fork, will merge.

[ursabot]

Benchmark runs are scheduled for baseline = 669009e and contender = 6d9a6a4. 6d9a6a4 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Failed ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Finished ⬇️0.37% ⬆️0.0%] test-mac-arm
[Finished ⬇️0.27% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.18% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Failed] 6d9a6a4a ec2-t3-xlarge-us-east-2
[Finished] 6d9a6a4a test-mac-arm
[Finished] 6d9a6a4a ursa-i9-9960x
[Finished] 6d9a6a4a ursa-thinkcentre-m75q
[Failed] 669009ed ec2-t3-xlarge-us-east-2
[Finished] 669009ed test-mac-arm
[Finished] 669009ed ursa-i9-9960x
[Finished] 669009ed ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java