fix install error

fix

```
value.go:28:1: error: 'g_value_get_char' is deprecated (declared at /usr/include/glib-2.0/gobject/gvaluetypes.h:184): Use 'g_value_get_schar' instead [-Werror=deprecated-declarations]
value.go:40:1: error: 'g_value_set_char' is deprecated (declared at /usr/include/glib-2.0/gobject/gvaluetypes.h:181): Use 'g_value_set_schar' instead [-Werror=deprecated-declarations]
cc1: all warnings being treated as errors
```