compile: don't #include unnecessary stuff

Time from base/time.hh has a name clash with Time from Ruby's
TypeDefines.hh.  Eventually Ruby's Time should go away, so instead of
fixing this properly just try to avoid the clash.

src/base/stats/visit.hh

@@ -31,11 +31,6 @@
 #ifndef __BASE_STATS_VISIT_HH__
 #define __BASE_STATS_VISIT_HH__
 
-#include <string>
-
-#include "base/time.hh"
-#include "base/types.hh"
-
 namespace Stats {
 
 class Info;