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/util/sort.c b/tools/perf/util/sort.c
index fe4fd7b..5762ae4 100644
--- a/tools/perf/util/sort.c
+++ b/tools/perf/util/sort.c
@@ -6,6 +6,7 @@
 #include "hist.h"
 #include "comm.h"
 #include "symbol.h"
+#include "thread.h"
 #include "evsel.h"
 #include "evlist.h"
 #include "strlist.h"