1 files changed, 1 insertions, 0 deletions
diff --git a/util/util.h b/util/util.h
index 8c5d922..7dfab93 100644
--- a/util/util.h
+++ b/util/util.h
@@ -31,6 +31,7 @@
 #include <utility>
 #include <set>
 #include <atomic>
+#include <mutex>        // For std::call_once
 
 // Use std names.
 using std::set;