Removed duplicated includes

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Closes #11775

module/os/linux/zfs/arc_os.c

@@ -38,7 +38,6 @@
 #include <sys/vdev_trim.h>
 #include <sys/vdev_impl.h>
 #include <sys/dsl_pool.h>
-#include <sys/zio_checksum.h>
 #include <sys/multilist.h>
 #include <sys/abd.h>
 #include <sys/zil.h>