HID: prodikeys: remove a redundant forward declaration of struct pcmidi_snd

Signed-off-by: NAxel Lin <axel.lin@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

drivers/hid/hid-prodikeys.c

@@ -44,8 +44,6 @@ struct pk_device {
 	struct pcmidi_snd	*pm; /* pcmidi device context */
 };
 
-struct pcmidi_snd;
-
 struct pcmidi_sustain {
 	unsigned long		in_use;
 	struct pcmidi_snd	*pm;