Suppress a bogus warning in MSVC

[phprus]

<source_location> and <ranges> is C++20 heades (FMT_CPLUSPLUS > 201703L), not C++17 (FMT_CPLUSPLUS >= 201703L).

Warnings:

The contents of <ranges> are available only with C++20 or later.
The contents of <source_location> are available only with C++20 consteval support.

[vitaut]

Thanks for the PR.

[vitaut]

Merged, thanks!