Fix to compile with newer Visual C++, such as 16.11, and

/Zc:implicitNoexcept-.
Otherwise gets:
  Error C2694 'override': overriding virtual function
  has less restrictive exception specification than base
  class virtual member function 'base'

  Similar changes are being made e.g.:
    boostorg/json#636

  And proposed here:
    boostorg/iostreams#136

I grant there there could be more of this.
These two are just enough for our codebase.

include/boost/format/alt_sstream.hpp

@@ -138,6 +138,7 @@ namespace boost {
             typedef basic_altstringbuf<Ch, Tr, Alloc>   stringbuf_t;
         public:
             typedef Alloc  allocator_type;
+            ~basic_oaltstringstream() noexcept override = default;
             basic_oaltstringstream() 
                 : pbase_type(new stringbuf_t), stream_t(pbase_type::member.get())
                 { }