video: of_display_timing.h: Declare 'display_timing'

Commit ffa3fd21 ("videomode: implement public of_get_display_timing()") causes
the following build warning:

include/video/of_display_timing.h:18:10: warning: 'struct display_timing' declared inside parameter list [enabled by default]
include/video/of_display_timing.h:18:10: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

Declare 'display_timing' to avoid the build warning.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

include/video/of_display_timing.h

@@ -10,6 +10,7 @@
 #define __LINUX_OF_DISPLAY_TIMING_H
 
 struct device_node;
+struct display_timing;
 struct display_timings;
 
 #define OF_USE_NATIVE_MODE -1