fix constructor of string_builtin_functiont

The default constructor cannot exist; marking it as 'delete' re-enables
compilation with clang.

Author: Daniel Kroening

src/solvers/strings/string_builtin_function.h

@@ -19,6 +19,7 @@ class string_constraint_generatort;
 class string_builtin_functiont
 {
 public:
+  string_builtin_functiont() = delete;
   string_builtin_functiont(const string_builtin_functiont &) = delete;
   virtual ~string_builtin_functiont() = default;
 
@@ -60,9 +61,6 @@ class string_builtin_functiont
     return true;
   }
 
-private:
-  string_builtin_functiont() = default;
-
 protected:
   array_poolt &array_pool;