Update os.h

include/spdlog/details/os.h

@@ -210,7 +210,7 @@ inline size_t filesize(FILE *f)
 #if defined(_WIN32) && !defined(__CYGWIN__)
     int fd = _fileno(f);
 #if _WIN64 // 64 bits
-    long long ret = _filelengthi64(fd);
+    __int64 ret = _filelengthi64(fd);
     if (ret >= 0)
     {
         return static_cast<size_t>(ret);