Prelude: cleanup compile warnings about unused variables

Harmless warnings, but this unnecessary noise hurts my eyes... :)

	tracked.cpp:66:37: warning: unused parameter ‘s’
	     void * op_new(std::size_t const s, bool const array, void * const r, char const * const name) {
	                                     ^
	In file included from prelude/prelude.hpp:14:0:
	prelude/type_strings.hpp:228:100: warning: unused parameter ‘v’
	   template <> struct group_list_desc<tlists::tlist<> > { static void s (std::vector<std::string> & v) {} };
	                                                                                                    ^