mingw32 cause compilation failure when -Wconversion is enabled while

cross compiling for win32 application using MXE environment because
long unsigned int != size_t (aka long long unsigned int)

include/fmt/base.h

@@ -727,7 +727,8 @@ class basic_specs {
   char fill_data_[max_fill_size] = {' '};
 
   FMT_CONSTEXPR void set_fill_size(size_t size) {
-    data_ = (data_ & ~fill_size_mask) | (size << fill_size_shift);
+    data_ = static_cast<unsigned long>((data_ & ~fill_size_mask) |
+                                       (size << fill_size_shift));
   }
 
  public: