Input: imx_keypad - add missing linux/input.h include

We are going to clean up matrix_keymap.h from unnecessary includes,
so the driver needs to include API that it uses directly.

Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/keyboard/imx_keypad.c

···
 #include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/err.h>
+#include <linux/input.h>
 #include <linux/input/matrix_keypad.h>
 #include <linux/interrupt.h>
 #include <linux/io.h>