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>

arch/x86/events/intel/p4.c

@@ -24,7 +24,7 @@ struct p4_event_bind {
 	unsigned int escr_msr[2];		/* ESCR MSR for this event */
 	unsigned int escr_emask;		/* valid ESCR EventMask bits */
 	unsigned int shared;			/* event is shared across threads */
-	char cntr[2][P4_CNTR_LIMIT];		/* counter index (offset), -1 on absence */
+	signed char cntr[2][P4_CNTR_LIMIT];	/* counter index (offset), -1 on absence */
 };
 
 struct p4_pebs_bind {