Fix Include Guards Copied From Other Headers

TriangleMeshDeal.hpp reused Timer.hpp's guard and CloneDialog.hpp reused
SingleChoiceDialog.hpp's, so including both headers of a pair silently
dropped the second one. Use #pragma once instead.

src/libslic3r/TriangleMeshDeal.hpp

@@ -1,5 +1,4 @@
-#ifndef libslic3r_Timer_hpp_
-#define libslic3r_Timer_hpp_
+#pragma once
 
 #include "TriangleMesh.hpp"
 
@@ -10,5 +9,3 @@ public:
     static TriangleMesh smooth_triangle_mesh(const TriangleMesh &mesh,bool& ok);
 };
 } // namespace Slic3r
-
-#endif // libslic3r_Timer_hpp_

src/slic3r/GUI/CloneDialog.hpp

@@ -1,5 +1,4 @@
-#ifndef slic3r_GUI_SingleChoice_hpp_
-#define slic3r_GUI_SingleChoice_hpp_
+#pragma once
 
 #include "GUI_Utils.hpp"
 #include "Plater.hpp"
@@ -33,5 +32,3 @@ private:
 
 };
 }} // namespace Slic3r::GUI
-
-#endif