pwm: Drop unused forward declaration from pwm.h

The declaration was necessary until commit cc2d2247 ("pwm: Drop
per-chip dbg_show callback").
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

include/linux/pwm.h

@@ -6,8 +6,6 @@
 #include <linux/mutex.h>
 #include <linux/of.h>
 
-struct seq_file;
-
 struct pwm_chip;
 
 /**