Export assert_fail with FMT_API. This fixes dll build.

[egorpugin]

See #1445

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

[vitaut]

I think FMT_API is not needed here.

[egorpugin]

Oh, maybe. First, I thought header goes to users, but it seems it is fmt internal stuff?

[egorpugin]

Yep, tested, works without it.

[vitaut]

It's for the opt-in header-only mode and effectively an internal header.

[vitaut]

Thanks!

[kayaalp]

I just pulled the most recent build 7.0.3, but still see the same problem mentioned here.

1>------ Rebuild All started: Project: TMP, Configuration: Debug Win32 ------
1>Source1.cpp
1>Source1.obj : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits,class std::allocator > __cdecl fmt::v7::detail::vformat(class fmt::v7::basic_string_view,struct fmt::v7::format_args)" (?vformat@detail@v7@fmt@@ya?AV?$basic_string@DU?$char_traits@D@std@@v?$allocator@D@2@@std@@v?$basic_string_view@D@23@Uformat_args@23@@z) referenced in function "class std::basic_string<char,struct std::char_traits,class std::allocator > __cdecl fmt::v7::format<char [11],char const (&)[6],char>(char const (&)[11],char const (&)[6])" (??$format@$$BY0L@DAAY05$$CBDD@v7@fmt@@ya?AV?$basic_string@DU?$char_traits@D@std@@v?$allocator@D@2@@std@@AAY0L@$$CBDAAY05$$CBD@Z)
1>C:\Users<user>\source\repos\TMP\Debug\TMP.exe : fatal error LNK1120: 1 unresolved externals
1>Done building project "TMP.vcxproj" -- FAILED.
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========