Revert "Removed unused <mutex> header include"

This reverts commit 87db424e45.
It was meant for the major branch.

src/exec.cpp

@@ -21,6 +21,7 @@
 #include <algorithm>
 #include <map>
 #include <memory>
+#include <mutex>
 #include <string>
 #include <type_traits>
 #include <vector>