With CONFIG_SERIO=m, we get a build error for the rmi4-f03 driver,
added in linux-4.10:

warning: (HID_RMI) selects RMI4_F03 which has unmet direct dependencies (!UML && INPUT && RMI4_CORE && (SERIO=y || RMI4_CORE=SERIO))
drivers/input/built-in.o: In function `rmi_f03_attention':
rmi_f03.c:(.text+0xcfe0): undefined reference to `serio_interrupt'
rmi_f03.c:(.text+0xd055): undefined reference to `serio_interrupt'
drivers/input/built-in.o: In function `rmi_f03_remove':
rmi_f03.c:(.text+0xd115): undefined reference to `serio_unregister_port'
drivers/input/built-in.o: In function `rmi_f03_probe':
rmi_f03.c:(.text+0xd209): undefined reference to `__serio_register_port'

An earlier patch tried to fix this, but missed the HID_RMI driver that
does a 'select' on the F03 backend.

This adds a hidden Kconfig symbol that enforces 'serio' to be enabled
when RMI4-F03 is, which covers all cases.

Fixes: d7ddad0a ("Input: synaptics-rmi4 - fix F03 build error when serio is module")
Fixes: c5e8848f ("Input: synaptics-rmi4 - add support for F03")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>