Fix no-Python build

A build without Python will currently fail, because
selftests::test_python uses gdb_python_initialized, which is only
conditionally defined.

This patch fixes the build by making test_python also be conditionally
defined.  I chose this approach because the selftest will fail if
Python is not enabled, so it didn't seem useful to leave it defined.

gdb/python/python.c

@@ -1878,8 +1878,6 @@ do_start_initialization ()
   return true;
 }
 
-#endif /* HAVE_PYTHON */
-
 #if GDB_SELF_TEST
 namespace selftests {
 
@@ -1921,6 +1919,8 @@ test_python ()
 } // namespace selftests
 #endif /* GDB_SELF_TEST */
 
+#endif /* HAVE_PYTHON */
+
 /* See python.h.  */
 cmd_list_element *python_cmd_element = nullptr;
 
@@ -2020,9 +2020,11 @@ python executable."),
 				&user_set_python_list,
 				&user_show_python_list);
 
+#ifdef HAVE_PYTHON
 #if GDB_SELF_TEST
   selftests::register_test ("python", selftests::test_python);
 #endif /* GDB_SELF_TEST */
+#endif /* HAVE_PYTHON */
 }
 
 #ifdef HAVE_PYTHON