Include <filesystem> only if FMT_CPP_LIB_FILESYSTEM is set (#4258)

This change results out of necessity since the Nintendo Switch console
SDK does not support `std::filesystem`. The SDK still provides the
`<filesystem>` header, but with an `#error` directive, effectively
breaking any build that includes `<filesystem>`

Because `<filesystem>` is present, `FMT_HAS_INCLUDE` is insufficient
here. With this change and `FMT_CPP_LIB_FILESYSTEM` in place, one can
define `FMT_CPP_LIB_FILESYSTEM=0` to work around this issue.

This assumes that `<filesystem>` can be included (without warnings) if
`FMT_CPP_LIB_FILESYSTEM` is set. If this is not the case, fmt would be
broken even before this change as `std::filesystem::path` is used
without the accompanying header.

include/fmt/std.h

@@ -27,9 +27,6 @@
 
 // Check FMT_CPLUSPLUS to suppress a bogus warning in MSVC.
 #  if FMT_CPLUSPLUS >= 201703L
-#    if FMT_HAS_INCLUDE(<filesystem>)
-#      include <filesystem>
-#    endif
 #    if FMT_HAS_INCLUDE(<variant>)
 #      include <variant>
 #    endif
@@ -79,6 +76,7 @@
 #endif
 
 #if FMT_CPP_LIB_FILESYSTEM
+#  include <filesystem>
 FMT_BEGIN_NAMESPACE
 
 namespace detail {