Mark new functions with FMT_API (#808)

include/fmt/format.h

@@ -348,11 +348,11 @@ inline fp operator-(fp x, fp y) {
 
 // Computes an fp number r with r.f = x.f * y.f / pow(2, 64) rounded to nearest
 // with half-up tie breaking, r.e = x.e + y.e + 64. Result may not be normalized.
-fp operator*(fp x, fp y);
+FMT_API fp operator*(fp x, fp y);
 
 // Returns cached power (of 10) c_k = c_k.f * pow(2, c_k.e) such that its
 // (binary) exponent satisfies min_exponent <= c_k.e <= min_exponent + 3.
-fp get_cached_power(int min_exponent, int &pow10_exponent);
+FMT_API fp get_cached_power(int min_exponent, int &pow10_exponent);
 
 template <typename Allocator>
 typename Allocator::value_type *allocate(Allocator& alloc, std::size_t n) {