spi: loopback-test: fix compile error on x86

Fix compile error caused by missing vmalloc() definition
on x86 (and maybe other platforms) by including vmalloc.h.
Signed-off-by: NFrode Isaksen <fisaksen@baylibre.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/spi-loopback-test.c

@@ -25,6 +25,7 @@
 #include <linux/module.h>
 #include <linux/of_device.h>
 #include <linux/printk.h>
+#include <linux/vmalloc.h>
 #include <linux/spi/spi.h>
 
 #include "spi-test.h"