Fix a warning about long long.

format.h

@@ -66,6 +66,7 @@
 # pragma warning(push)
 # pragma warning(disable: 4521)
 #endif
+
 namespace fmt {
 
 namespace internal {
@@ -939,6 +940,8 @@ class BasicFormatter {
   int num_open_braces_;
   int next_arg_index_;
 
+  typedef unsigned long long ULongLong;
+
   friend class internal::FormatterProxy<Char>;
 
   // Forbid copying from a temporary as in the following example: