libutil: add in-place construction to Sync

only then can we Sync<> non-movable types.

Change-Id: I7835cdaf9fd2b90c842c9e054942524f36515d5a

lix/libutil/sync.hh

@@ -10,6 +10,7 @@
 #include <condition_variable>
 #include <cassert>
 #include <optional>
+#include <utility>
 
 namespace nix {
 
@@ -44,6 +45,9 @@ public:
     Sync(const T & data) : data(data) { }
     Sync(T && data) noexcept : data(std::move(data)) { }
 
+    template<typename ... Args>
+    Sync(std::in_place_t, Args &&... args) : data(std::forward<Args>(args)...) { }
+
     class Lock
     {
     protected: