Merge pull request #1120 from casperevan/patch-1

Fix missing include for null_mutex in rotating_file_sink.h

include/spdlog/sinks/rotating_file_sink.h

@@ -5,6 +5,7 @@
 
 #include "spdlog/sinks/base_sink.h"
 #include "spdlog/details/file_helper.h"
+#include "spdlog/details/null_mutex.h"
 #include "spdlog/details/synchronous_factory.h"
 
 #include <chrono>