Fix syslog_sink and example

example/example.cpp

@@ -233,7 +233,7 @@ void err_handler_example() {
     #include "spdlog/sinks/syslog_sink.h"
 void syslog_example() {
     std::string ident = "spdlog-example";
-    auto syslog_logger = spdlog::create<syslog_sink_mt>("syslog", ident, 0);
+    auto syslog_logger = spdlog::create<syslog_sink_mt>("syslog", ident, LOG_PID);
     syslog_logger->warn("This is warning that will end up in syslog.");
 }
 #endif

include/spdlog/sinks/syslog_sink.h

@@ -9,6 +9,7 @@
 
 #include <array>
 #include <string>
+#include <mutex>
 
 namespace spdlog {
 namespace sinks {