Fix -Wunused-private-field when building with clang-cl on Windows

[hebasto]

Below are examples of such warnings:

> cmake -B build -T ClangCL -DSUBPROCESS_TESTS=ON
> cmake --build build
...
C:\Users\hebas\dev\cpp-subprocess\cpp-subprocess/subprocess.hpp(1190,10): warning : private field 'parent_' is not used [-Wunused-private-field] [C:\Users\hebas\dev\cpp-subprocess\build\test\test_subprocess.vcxproj]
C:\Users\hebas\dev\cpp-subprocess\cpp-subprocess/subprocess.hpp(1191,7): warning : private field 'err_wr_pipe_' is not used [-Wunused-private-field] [C:\Users\hebas\dev\cpp-subprocess\build\test\test_subprocess.vcxproj]

Ported from bitcoin/bitcoin#34385.