visibility attribute not supported warnings on mingw64

[mizvekov]

Compiling with GCC 10.1 for windows (mingw64) produces a bunch of these kinds of warnings:

fmt/format-inl.h: In member function '__dt_del ':
fmt\format-inl.h(213,18): warning G2331D7DD: visibility attribute not supported in this configuration; ignored [-Wattributes]
    213 | FMT_API FMT_FUNC format_error::~format_error() FMT_NOEXCEPT = default;

So yeah, visibility attribute is not supported for windows target.

Note: __has_atttribute is not an option to solve this, it will still evaluate to true.

[vitaut]

Removed the non-portable attribute in 3135d95. Thanks for reporting.