Added a FMT_STRING wrapper for system_error() call.

[madmaxoft]

Without this change, using fmt with FMT_ENFORCE_COMPILE_STRING in MSVC 2019 results in failed compilation in fmt itself:

10>C:\...\fmt\include\fmt\core.h(2934,39): error C2338: FMT_ENFORCE_COMPILE_STRING requires all format strings to use FMT_STRING. (compiling source file C:\...\frmNearCrossings.cpp)
10>C:\...\fmt\include\fmt\core.h(3077): message : see reference to function template instantiation 'void fmt::v8::detail::check_format_string<,char[21],0>(const S (&))' being compiled
10>        with
10>        [
10>            S=char [21]
10>        ] (compiling source file C:\...\frmNearCrossings.cpp)
10>C:\...\fmt\include\fmt\format-inl.h(83): message : see reference to function template instantiation 'fmt::v8::basic_format_string<char>::basic_format_string<char[21],0>(const S (&))' being compiled
10>        with
10>        [
10>            S=char [21]
10>        ] (compiling source file C:\...\frmNearCrossings.cpp)
10>C:\...\fmt\include\fmt\format-inl.h(83): message : see reference to function template instantiation 'fmt::v8::basic_format_string<char>::basic_format_string<char[21],0>(const S (&))' being compiled
10>        with
10>        [
10>            S=char [21]
10>        ] (compiling source file C:\...\frmNearCrossings.cpp)

(I'm using fmt as a git submodule in another project, using CMake for build and adding target_compile_definitions(fmt-header-only INTERFACE FMT_ENFORCE_COMPILE_STRING=1) to my CMakeLists.txt

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

[vitaut]

Thanks for the PR.