toshiba_acpi: Add dependency on SERIO_I8042

Configuring this option as a module with ACPI_TOSHIBA built-in
results in the following errors:

   drivers/built-in.o: In function `toshiba_acpi_remove':
>> toshiba_acpi.c:(.text+0x314bb0): undefined reference to `i8042_remove_filter'
   drivers/built-in.o: In function `toshiba_acpi_add':
>> toshiba_acpi.c:(.devinit.text+0xb822): undefined reference to `i8042_install_filter'
>> toshiba_acpi.c:(.devinit.text+0xb98b): undefined reference to `i8042_remove_filter'

Add a dependency to prevent ACPI_TOSHIBA from being built-in when
SERIO_I8042=m.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Azael Avalos <coproscefalo@gmail.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>