add Key for muting Mic

 * this exists on most IBM/Lenovo Thinkpads

dump of `xev`:

KeyPress event, serial 34, synthetic NO, window 0x2e00001,
    root 0x97, subw 0x0, time 306796922, (668,272), root:(670,293),
    state 0x0, keycode 198 (keysym 0x1008ffb2, XF86AudioMicMute), same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False
KeyRelease event, serial 34, synthetic NO, window 0x2e00001,
    root 0x97, subw 0x0, time 306796922, (668,272), root:(670,293),
    state 0x0, keycode 198 (keysym 0x1008ffb2, XF86AudioMicMute), same_screen YES,
    XLookupString gives 0 bytes:
    XFilterEvent returns: False