fix compile error

In file included from include/sig.hpp:6:0,
                 from tests/timer.cpp:1:
include/signaltimer.hpp: In instantiation of ‘class sig::SignalTimer<int>’:
tests/timer.cpp:6:27:   required from here
include/signaltimer.hpp:28:11: error: looser throw specifier for ‘sig::SignalTimer<Params>::~SignalTimer() [with Params = {int}]’
In file included from include/sig.hpp:4:0,
                 from tests/timer.cpp:1:
include/signal.hpp:62:11: error:   overriding ‘sig::Signal<Params>::~Signal() noexcept (true) [with Params = {int}]’
make: *** [timer.test] Error 1