Add 2 more constexprs to fix compile error (#4065)

include/fmt/base.h

@@ -621,11 +621,12 @@ namespace detail {
 // to it, deducing Char. Explicitly convertible types such as the ones returned
 // from FMT_STRING are intentionally excluded.
 template <typename Char, FMT_ENABLE_IF(is_char<Char>::value)>
-auto to_string_view(const Char* s) -> basic_string_view<Char> {
+constexpr auto to_string_view(const Char* s) -> basic_string_view<Char> {
   return s;
 }
 template <typename T, FMT_ENABLE_IF(is_std_string_like<T>::value)>
-auto to_string_view(const T& s) -> basic_string_view<typename T::value_type> {
+constexpr auto to_string_view(const T& s)
+    -> basic_string_view<typename T::value_type> {
   return s;
 }
 template <typename Char>