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-mem.c b/tools/perf/builtin-mem.c
index 2e5be1d..e001c02 100644
--- a/tools/perf/builtin-mem.c
+++ b/tools/perf/builtin-mem.c
@@ -12,6 +12,7 @@
 #include "util/data.h"
 #include "util/mem-events.h"
 #include "util/debug.h"
+#include "util/symbol.h"
 
 #define MEM_OPERATION_LOAD	0x1
 #define MEM_OPERATION_STORE	0x2