Add explicit pthread.h include
diff --git a/include/benchmark/benchmark.h b/include/benchmark/benchmark.h
index 293678c..5c42581 100644
--- a/include/benchmark/benchmark.h
+++ b/include/benchmark/benchmark.h
@@ -139,6 +139,7 @@
 
 #include <functional>
 #include <memory>
+#include <pthread.h>
 #include <string>
 #include <vector>