Add template deduction guide for scoped_guard (#36)

Author: AutonomicPerfectionist

include/gtl/utils.hpp

@@ -85,6 +85,11 @@ class scoped_guard {
     F    unset_;
 };
 
+#if __cpp_deduction_guides >= 201703
+template<class F>
+scoped_guard(F f) -> scoped_guard<F>;
+#endif
+
 // ---------------------------------------------------------------------------
 // An object which assigns a value to a variable in its constructor, and resets
 // the previous its destructor