viewer: warning fix when using gtk-2.14.
viewer: warning fix when using gtk-2.14.

use-action-appearance is defined since 2.16.

AravisProject/aravis,AravisProject/aravis,AnilRamachandran/aravis,AravisProject/aravis,AnilRamachandran/aravis,AravisProject/aravis,AnilRamachandran/aravis,AnilRamachandran/aravis,AnilRamachandran/aravis,AravisProject/aravis