Fix warning C4251: class fmt::v8::file needs to have dll-interface

[dalboris]

These changes fix #2796 by exporting the whole class fmt::v8::file instead of exporting all its individual member functions.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

There are some link errors: https://github.com/fmtlib/fmt/runs/5442603541

[dalboris]

@vitaut Looking into it

[dalboris]

The problem was that fmt was missing in this line in test/CMakeLists :

target_link_libraries(test-main gtest fmt)

Without the fmt dependency, the test-main target didn't inherit the FMT_SHARED compile definition defined L272 of fmt/CMakeLists:

target_compile_definitions(fmt PRIVATE FMT_EXPORT INTERFACE FMT_SHARED)

Therefore, when compiling test-main, the macro FMT_API resolved to nothing, so test-main didn't know it had to import existing symbols (such as fmt::file::file() and others) and instead re-defined these symbols.

Later, when compiling a test (for exemple, os-test.cc), the test does depend on fmt, so it inherits FMT_SHARED, so FMT_API resolves to __declspec(dllimport), so it correctly imports the symbols from the fmt shared lib, which conflicts with the redefined symbols in test-main.

[dalboris]

Shout out to @PixelRick who found the problem after a lot of head scratching together.

[vitaut]

Thank you