clients: Initialize label in keyboard handling code
clients: Initialize label in keyboard handling code

Quells warning:
  clients/keyboard.c: In function ‘keyboard_handle_key.isra.5’:
  clients/keyboard.c:556:11: warning: ‘label’ may be used uninitialized in
  this function [-Wuninitialized]

Signed-off-by: Bryce Harrington <dd437fc32dcda8cbb48d47baa844ad777493d02b@samsung.com>

sir-murray/weston,kwm81/weston,jonnylamb/weston,udoprog/weston,jonnylamb/weston,udoprog/weston,giucam/weston,Gnurou/weston,eyolfson/weston,xorgy/weston,Gnurou/weston,mchalupa/weston,krezovic/weston,sir-murray/weston,Tarnyko/weston-xdg_surface_present,Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,krezovic/weston,Fantu/compositor-spice,xorgy/weston,mchalupa/weston,Fantu/compositor-spice,kwm81/weston,eyolfson/weston,giucam/weston