perf stat: Remove needless headers from stat.h

Just a forward declaration for 'struct timespec' is needed, ditch the
rest.

Cc: Adrian Hunter <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/stat.h

···
 #include <linux/types.h>
 #include <stdio.h>
 #include <sys/types.h>
-#include <sys/time.h>
 #include <sys/resource.h>
-#include <sys/wait.h>
 #include "rblist.h"
-#include "perf.h"
 #include "event.h"
+
+struct timespec;
 
 struct stats {
 	double n, mean, M2;