Fix missing explicit for single-argument constructors

Author: LT2H

source/to_cpp1.h

@@ -6537,7 +6537,7 @@ class cppfront
                                 func->is_constructor()
                                 && !func->is_constructor_with_that()
                                 && generating_assignment_from != &n
-                                && func->parameter_count() < 2
+                                && func->parameter_count() < 3
                                 )
                             {
                                 prefix += "explicit ";