fix gcc 4.8 compile warning

include/spdlog/common.h

@@ -308,6 +308,7 @@ struct file_event_handlers
     std::function<void(const filename_t &filename, std::FILE *file_stream)> after_open;
     std::function<void(const filename_t &filename, std::FILE *file_stream)> before_close;
     std::function<void(const filename_t &filename)> after_close;
+    file_event_handlers() : before_open{nullptr}, after_open{nullptr}, before_close{nullptr}, after_close{nullptr} {}
 };
 
 namespace details {