Create no-op Window pathlocks implementation

This keeps the call sites simple, eventually this should be filled in.

src/libstore/windows/pathlocks-impl.hh

@@ -0,0 +1,2 @@
+#pragma once
+///@file Needed because Unix-specific counterpart

src/libstore/windows/pathlocks.cc

@@ -0,0 +1,16 @@
+#include "logging.hh"
+#include "pathlocks.hh"
+
+namespace nix {
+
+bool PathLocks::lockPaths(const PathSet & _paths, const std::string & waitMsg, bool wait)
+{
+    return true;
+}
+
+void PathLocks::unlock()
+{
+    warn("PathLocks::unlock: not yet implemented");
+}
+
+}