perf/x86: Make struct p4_event_bind::cntr signed array

perf/x86: Make struct p4_event_bind::cntr signed array

struct p4_event_bind::cntr[][] should be signed because of
the following code:

	int i, j;
        for (i = 0; i < P4_CNTR_LIMIT; i++) {
  --->          j = bind->cntr[thread][i];
                if (j != -1 && !test_bit(j, used_mask))
                        return j;
        }

Making this member unsigned will make "j" 255 and fail "j != -1"
comparison.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>