Suppress deprecated warnings for g_thread_init() and g_type_init()

    milter-core.c: In function 'milter_init':
    milter-core.c:79:9: warning: 'g_thread_init' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:260) [-Wdeprecated-declarations]
             g_thread_init(NULL);
             ^
    milter-core.c:81:5: warning: 'g_type_init' is deprecated (declared at /usr/include/glib-2.0/gobject/gtype.h:669) [-Wdeprecated-declarations]
         g_type_init();
         ^

GitHub: see #32