perf stat: Remove needless headers from stat.h

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

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-6shdqw801oqe7ax6r307k27r@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/stat.h

@@ -5,13 +5,12 @@
 #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;
 	u64 max, min;