Fix linker errors using fmt as shared library in MSVC

include/fmt/core.h

@@ -405,7 +405,7 @@ struct error_handler {
   FMT_CONSTEXPR error_handler(const error_handler &) {}
 
   // This function is intentionally not constexpr to give a compile-time error.
-  void on_error(const char *message);
+  FMT_API void on_error(const char *message);
 };
 
 // Formatting of wide characters and strings into a narrow output is disallowed:

include/fmt/format.h

@@ -905,7 +905,7 @@ class add_thousands_sep {
 };
 
 template <typename Char>
-Char thousands_sep(locale_provider *lp);
+FMT_API Char thousands_sep(locale_provider *lp);
 
 // Formats a decimal unsigned integer value writing into buffer.
 // thousands_sep is a functor that is called after writing each char to