fixed compilation error in 2013 (does not support thread_local keyword)

include/spdlog/details/os.h

@@ -315,8 +315,12 @@ inline size_t _thread_id()
 //Return current thread id as size_t (from thread local storage)
 inline size_t thread_id()
 {
+#if defined(_MSC_VER) && (_MSC_VER < 1900)
+    return _thread_id();
+#else
     static thread_local const size_t tid = _thread_id();
     return tid;
+#endif
 }