minikconf: print error entirely on stderr

While debugging an invalid configuration, I noticed that the clauses debug
ends up on stderr but the header ("The following clauses were found..."
ends up on stdout.  This makes the contents of meson-logs/meson-log.txt
a bit confusing.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/minikconf.py

@@ -112,7 +112,7 @@ class KconfigData:
         def set_value(self, val, clause):
             self.clauses_for_var.append(clause)
             if self.has_value() and self.value != val:
-                print("The following clauses were found for " + self.name)
+                print("The following clauses were found for " + self.name, file=sys.stderr)
                 for i in self.clauses_for_var:
                     print("    " + str(i), file=sys.stderr)
                 raise KconfigDataError('contradiction between clauses when setting %s' % self)