SMT2 backend: add missing whitespace

This adds a missing whitespace in the SMT2 output.  The SMT2 formula parses
correctly without the whitespace, so this is cosmetic.

Author: kroening

src/solvers/smt2/smt2_conv.cpp

@@ -4839,7 +4839,7 @@ void smt2_convt::find_symbols(const exprt &expr)
 
       for(std::size_t i=0; i<tmp.operands().size(); i++)
       {
-        out << "(assert (= (select " << id;
+        out << "(assert (= (select " << id << ' ';
         convert_expr(from_integer(i, array_type.size().type()));
         out << ") "; // select
         convert_expr(tmp.operands()[i]);