Fix failing MSVC build on GitHub Actions

[striezel]

Fixes the build failure with MSVC on Windows 2022.

See https://github.com/martinmoene/expected-lite/actions/runs/13823506911/job/38673977228?pr=79 for the failure. Boils down to:

D:\a\expected-lite\expected-lite\test\expected.t.cpp(2245,13): error C3861: 'assert': identifier not found [D:\a\expected-lite\expected-lite\build\test\expected-lite-cpplatest.t.vcxproj]

So adding the header <cassert> which declares the assert macro should fix the error.