Include what we use.

Change-Id: If510431d3d445de94f781b4fdec2e1b445f7c294
diff --git a/base/logging.cpp b/base/logging.cpp
index 8bfb204..38ee2af 100644
--- a/base/logging.cpp
+++ b/base/logging.cpp
@@ -18,6 +18,7 @@
 
 #include <iostream>
 #include <limits>
+#include <mutex>
 #include <sstream>
 #include <string>
 #include <vector>