Apply field sensitivity in symex_printf

This may be necessary for constant propagation of the format string.
An example of that is in regression/cbmc/printf1

Author: romainbrenguier

src/goto-symex/symex_builtin_functions.cpp

@@ -390,6 +390,7 @@ void goto_symext::symex_printf(
   else
   {
     clean_expr(*va_args, state, false);
+    *va_args = field_sensitivityt{}.apply(ns, state, *va_args, false);
     *va_args = state.rename(*va_args, ns).get();
     if(va_args->id() != ID_array)
     {