Fix gnulib fts usage.

* libls/cmp.c: Include fts_.h instead of fts.h.
* libls/ls.c: Likewise.
* libls/print.c: Likewise.
* libls/util.c: Likewise.

libls/cmp.c

@@ -54,7 +54,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 
-#include "fts.h"
+#include <fts_.h>
 #include <string.h>
 
 #include "ls.h"

libls/ls.c

@@ -57,7 +57,7 @@
 #include <sys/ioctl.h>
 
 #include <errno.h>
-#include "fts.h"
+#include <fts_.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

libls/print.c

@@ -56,7 +56,7 @@
 #include <sys/stat.h>
 
 #include <errno.h>
-#include "fts.h"
+#include <fts_.h>
 #include <grp.h>
 #include <pwd.h>
 #include <stdio.h>

libls/util.c

@@ -55,7 +55,7 @@
 #include <sys/stat.h>
 
 #include <ctype.h>
-#include "fts.h"
+#include <fts_.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>