staging: fix ste_rmi4 build, depends on INPUT

The synaptics_i2c_rmi4 driver uses input_*() interfaces,
so it should depend on INPUT to fix its build errors:

ERROR: "input_event" [drivers/staging/ste_rmi4/synaptics_i2c_rmi4.ko] undefined!
ERROR: "input_free_device" [drivers/staging/ste_rmi4/synaptics_i2c_rmi4.ko] undefined!
ERROR: "input_register_device" [drivers/staging/ste_rmi4/synaptics_i2c_rmi4.ko] undefined!
ERROR: "input_set_abs_params" [drivers/staging/ste_rmi4/synaptics_i2c_rmi4.ko] undefined!
ERROR: "input_allocate_device" [drivers/staging/ste_rmi4/synaptics_i2c_rmi4.ko] undefined!
ERROR: "input_unregister_device" [drivers/staging/ste_rmi4/synaptics_i2c_rmi4.ko] undefined!
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: naveen.gaddipati@stericsson.com, js.ha@stericsson.com
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/ste_rmi4/Kconfig

 config TOUCHSCREEN_SYNAPTICS_I2C_RMI4
 	tristate "Synaptics i2c rmi4 touchscreen"
-	depends on I2C
+	depends on I2C && INPUT
 	help
 	  Say Y here if you have a Synaptics RMI4 and
 	  want to enable support for the built-in touchscreen.