platform/x86: silead depends on I2C being built-in

The new driver cannot be a loadable module, so if I2C is loadable, we get this
link error:

drivers/platform/built-in.o: In function `silead_ts_dmi_init':
silead_dmi.c:(.init.text+0x2ef): undefined reference to `i2c_bus_type'

This makes the Kconfig dependency stricter to require I2C=y.

Fixes: 9eeda3897a85 ("platform/x86: add support for devices with Silead touchscreens")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>