include cleanup using iwyu

src/UpdateWalk.cxx

@@ -44,12 +44,8 @@
 #include <glib.h>
 
 #include <assert.h>
-#include <sys/types.h>
 #include <sys/stat.h>
-#include <unistd.h>
-#include <dirent.h>
 #include <string.h>
-#include <stdlib.h>
 #include <errno.h>
 
 bool walk_discard;