fix: missing dependency for WIN32

[coryan]

We were missing a direct dependency on Windows. It probably went
unnoticed because in our tests we get this dependency indirectly. It is
important for Conda support.

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This change is 

[google-cloud-cpp-bot]

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: f15fa5ef8e0d272934580dd32a77e6b776842792

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[codecov]

Codecov Report

Merging #7718 (f15fa5e) into main (cc5834b) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #7718   +/-   ##
=======================================
  Coverage   95.29%   95.30%           
=======================================
  Files        1256     1256           
  Lines      113697   113697           
=======================================
+ Hits       108350   108354    +4     
+ Misses       5347     5343    -4     

Impacted Files	Coverage Δ
.../cloud/storage/benchmarks/throughput_experiment.cc	74.87% <0.00%> (+0.50%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	86.18% <0.00%> (+0.65%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.51% <0.00%> (+2.19%)	⬆️

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