fix building of the accessx-status applet

I have been unable to build mate-applets since recent commits that added gio(desktopappinfo) to one of the applets. The relevant bit of my build log is:
```
Merging translations into org.mate.applets.AccessxStatusApplet.mate-panel-applet.
applet.c:29:33: fatal error: gio/gdesktopappinfo.h: No such file or directory
 #include <gio/gdesktopappinfo.h>
                                 ^
compilation terminated.
```

More importantly, this PR fixes the issue.