Fix compilation with clang-1205.0.22.9.

This fixes the following error:

`ISO C++ requires the name after '::~' to be found in the same scope as the name before '::~' [-Werror,-Wdtor-name]
sg::detail::scope_guard<Callback>::~scope_guard() noexcept´

scope_guard.hpp

@@ -149,7 +149,7 @@ noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value)
 
 ////////////////////////////////////////////////////////////////////////////////
 template<typename Callback>
-sg::detail::scope_guard<Callback>::~scope_guard() noexcept
+sg::detail::scope_guard<Callback>::~scope_guard<Callback>() noexcept
 {
   if(m_active)
     m_callback();