Fix unused parameter warnings.

In SignalHandling constructor, the empty signal vector is never used,
which triggers unused parameter warnings at least on Clang 3.2.

backward.hpp

@@ -2112,7 +2112,7 @@ private:
 
 class SignalHandling {
 public:
-	SignalHandling(const std::vector<int>& signals = std::vector<int>()) {}
+	SignalHandling(const std::vector<int>& signals = std::vector<int>()) { (void)signals; }
 	bool init() { return false; }
 };