perf: Depend on KERNEL_PERF_EVENTS

perf: Depend on KERNEL_PERF_EVENTS

The kernel needs to have PERF_EVENTS built otherwise we will run into
the following:

root@(none):/# perf top
perf_event_open(..., PERF_FLAG_FD_CLOEXEC) failed with unexpected error
89 (Function not implemented)
perf_event_open(..., 0) failed unexpectedly with error 89 (Function not
implemented)
Error:
The sys_perf_event_open() syscall returned with 89 (Function not
implemented) for event (cycles).
/bin/dmesg may provide additional information.
No CONFIG_PERF_EVENTS=y kernel support configured?

Make sure this functional dependency is captured.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>