fix: compiler warnings

~~~
In file included from include/ibex/ibex.h:27:0,
                 from /Users/soonhok/work/dreal/src/dsolvers/nra_solver.h:32,
                 from /Users/soonhok/work/dreal/src/opensmt/egraph/EgraphSolver.C:33:
include/ibex/ibex_Bsc.h:99:36: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
  static const double default_ratio();
                                    ^
In file included from include/ibex/ibex_CtcEmpty.h:15:0,
                 from include/ibex/ibex.h:37,
                 from /Users/soonhok/work/dreal/src/dsolvers/nra_solver.h:32,
                 from /Users/soonhok/work/dreal/src/opensmt/egraph/EgraphSolver.C:33:
include/ibex/ibex_Pdc.h: In static member function 'static bool ibex::Pdc::check_nb_var_pdc_list(const ibex::Array<ibex::Pdc>&)':
include/ibex/ibex_Pdc.h:66:12: warning: unused variable 'n' [-Wunused-variable]
   int i=1, n=l[0].nb_var;
            ^
In file included from include/ibex/ibex.h:65:0,
                 from /Users/soonhok/work/dreal/src/dsolvers/nra_solver.h:32,
                 from /Users/soonhok/work/dreal/src/opensmt/egraph/EgraphSolver.C:33:
include/ibex/ibex_Gradient.h: At global scope:
include/ibex/ibex_Gradient.h:104:58: warning: unused parameter 'x' [-Wunused-parameter]
     inline void trans_V_bwd(const ExprTrans&, ExprLabel& x,                 const ExprLabel& y) { /* nothing to do because x.g->v() is a reference to y.g->v()
                                                          ^
include/ibex/ibex_Gradient.h:104:78: warning: unused parameter 'y' [-Wunused-parameter]
     inline void trans_V_bwd(const ExprTrans&, ExprLabel& x,                 const ExprLabel& y) { /* nothing to do because x.g->v() is a reference to y.g->v()
                                                                              ^
In file included from include/ibex/ibex.h:116:0,
                 from /Users/soonhok/work/dreal/src/dsolvers/nra_solver.h:32,
                 from /Users/soonhok/work/dreal/src/opensmt/egraph/EgraphSolver.C:33:
include/ibex/ibex_CellHeapOptim.h:91:19: warning: unused parameter 'cell' [-Wunused-parameter]
   void push(Cell* cell) {};
~~~