Update gnome-icon-theme-symbolic to 3.12.0

After this version, gnome-icon-theme-symbolic is merged with
gnome-icon-theme and renamed to adwaita-icon-theme.

3.11.92
  - fix exported wifi and volume icons
  - fix epoxrted goa-panel
  - inode-directory mimetype icon
  - network offline status icons (bug #726321)
  - larger a/c icon (bug #701031)

3.11.91
  - fix export issues of audio-input-microphone (bug #724670)
  - media-view-subtitles (bug #722198)

3.11.5
  - refined ethernet/wired icons
  - missing device states for wired and cellular
  - outlines for some icons causing alpha compositing issues

3.11.2
  - network-wireless
  - action-unavailable fixes
  - grid alignment
  - avatar less blocky
  - new computer icons
  - places more consistent
  - updated network status icons (arrows)

(prlw1)