Fix unused parameter warnings.
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.

Manu343726/backward-cpp,Manu343726/backward-cpp,bombela/backward-cpp,Ghostist/backward-cpp,Ghostist/backward-cpp,m13253/backward-cpp,m13253/backward-cpp,bombela/backward-cpp,Manu343726/backward-cpp,bombela/backward-cpp