mtd: tests: Include <linux/prandom.h> instead of <linux/random.h>

mtd: tests: Include <linux/prandom.h> instead of <linux/random.h>

Substitute the inclusion of <linux/random.h> header with
<linux/prandom.h> to allow the removal of legacy inclusion
of <linux/prandom.h> from <linux/random.h>.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>