perf tools: Use just forward declarations for struct thread where possible

Removing various instances of unnecessary includes, reducing the maze of
header dependencies.

Link: http://lkml.kernel.org/n/tip-hwu6eyuok9pc57alookyzmsf@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/builtin-timechart.c b/tools/perf/builtin-timechart.c
index 38e2c43..4e2e616 100644
--- a/tools/perf/builtin-timechart.c
+++ b/tools/perf/builtin-timechart.c
@@ -29,6 +29,7 @@
 #include <linux/rbtree.h>
 #include <linux/time64.h>
 #include "util/symbol.h"
+#include "util/thread.h"
 #include "util/callchain.h"
 
 #include "perf.h"