Fix linkage errors when linking with a shared library (#2011)

include/fmt/format.h

@@ -1274,7 +1274,7 @@ template <typename T> struct decimal_fp {
   int exponent;
 };
 
-template <typename T> decimal_fp<T> to_decimal(T x) FMT_NOEXCEPT;
+template <typename T> FMT_API decimal_fp<T> to_decimal(T x) FMT_NOEXCEPT;
 }  // namespace dragonbox
 
 template <typename T>

include/fmt/os.h

@@ -378,7 +378,7 @@ struct ostream_params {
 static constexpr detail::buffer_size buffer_size;
 
 // A fast output stream which is not thread-safe.
-class ostream final : private detail::buffer<char> {
+class FMT_API ostream final : private detail::buffer<char> {
  private:
   file file_;