remove unused includes

This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.

src/libudev/libudev-queue.c

@@ -18,17 +18,12 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <stdio.h>
 #include <stdlib.h>
 #include <stddef.h>
 #include <unistd.h>
 #include <errno.h>
-#include <string.h>
-#include <limits.h>
-#include <sys/stat.h>
 #include <sys/inotify.h>
 
-#include "libudev.h"
 #include "libudev-private.h"
 
 /**