macintosh/via-pmu: Fix build failure when CONFIG_INPUT is disabled

drivers/macintosh/via-pmu-event.o: In function `via_pmu_event':
via-pmu-event.c:(.text+0x44): undefined reference to `input_event'
via-pmu-event.c:(.text+0x68): undefined reference to `input_event'
via-pmu-event.c:(.text+0x94): undefined reference to `input_event'
via-pmu-event.c:(.text+0xb8): undefined reference to `input_event'
drivers/macintosh/via-pmu-event.o: In function `via_pmu_event_init':
via-pmu-event.c:(.init.text+0x20): undefined reference to `input_allocate_device'
via-pmu-event.c:(.init.text+0xc4): undefined reference to `input_register_device'
via-pmu-event.c:(.init.text+0xd4): undefined reference to `input_free_device'
make[1]: *** [Makefile:1155: vmlinux] Error 1
make: *** [Makefile:350: __build_one_by_one] Error 2

Don't call into the input subsystem unless CONFIG_INPUT is built-in.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5edbe76ce68227f71e09af4614cc4c1bd61c7ec8.1649326292.git.fthain@linux-m68k.org