Input: stmpe-keypad - add missing of.h include

The driver is using of_property_read_u32() and therefore needs to include
of.h header.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/stmpe-keypad.c

@@ -9,6 +9,7 @@
 #include <linux/slab.h>
 #include <linux/input.h>
 #include <linux/interrupt.h>
+#include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/input/matrix_keypad.h>
 #include <linux/mfd/stmpe.h>