Added a FMT_STRING wrapper for system_error() call.

This enables the use of FMT_ENFORCE_COMPILE_STRING on MSVC 2019.

include/fmt/format-inl.h

@@ -80,7 +80,8 @@ FMT_FUNC void report_error(format_func func, int error_code,
 inline void fwrite_fully(const void* ptr, size_t size, size_t count,
                          FILE* stream) {
   size_t written = std::fwrite(ptr, size, count, stream);
-  if (written < count) FMT_THROW(system_error(errno, "cannot write to file"));
+  if (written < count)
+    FMT_THROW(system_error(errno, FMT_STRING("cannot write to file")));
 }
 
 #ifndef FMT_STATIC_THOUSANDS_SEPARATOR