xdiff: remove duplicate headers from xhistogram.c

8c912eea94 ("teach --histogram to diff", 2011-07-12) included them, but
were already part of xinclude.h

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/xdiff/xhistogram.c b/xdiff/xhistogram.c
index ec85f59..c7b35a9 100644
--- a/xdiff/xhistogram.c
+++ b/xdiff/xhistogram.c
@@ -42,8 +42,6 @@
  */
 
 #include "xinclude.h"
-#include "xtypes.h"
-#include "xdiff.h"
 
 #define MAX_PTR	UINT_MAX
 #define MAX_CNT	UINT_MAX