fix the following 'make includecheck' warning:

  Documentation/auxdisplay/cfag12864b-example.c: string.h is included more than once.
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/auxdisplay/cfag12864b-example.c

@@ -194,7 +194,6 @@ static void cfag12864b_blit(void)
  */
 
 #include <stdio.h>
-#include <string.h>
 
 #define EXAMPLES	6