media: i2c: adv748x: Provide slab.h headers for kzalloc/kfree

Certain combinations of kernel configuration fail to compile ADV748x
with errors on kzalloc and kfree:

drivers/media/i2c/adv748x/adv748x-core.c:693:10: error: implicit
declaration of function ‘kzalloc’
[-Werror=implicit-function-declaration]
  state = kzalloc(sizeof(struct adv748x_state), GFP_KERNEL);

Fix this by providing include/slab.h

Fixes: e261761c3662 ("media: i2c: adv748x: add adv748x driver")
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
diff --git a/drivers/media/i2c/adv748x/adv748x-afe.c b/drivers/media/i2c/adv748x/adv748x-afe.c
index ac05735..f268f73 100644
--- a/drivers/media/i2c/adv748x/adv748x-afe.c
+++ b/drivers/media/i2c/adv748x/adv748x-afe.c
@@ -13,6 +13,7 @@
 #include <linux/delay.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
+#include <linux/slab.h>
 #include <linux/v4l2-dv-timings.h>
 
 #include <media/v4l2-ctrls.h>
diff --git a/drivers/media/i2c/adv748x/adv748x-core.c b/drivers/media/i2c/adv748x/adv748x-core.c
index 4cb6729..0a827e3 100644
--- a/drivers/media/i2c/adv748x/adv748x-core.c
+++ b/drivers/media/i2c/adv748x/adv748x-core.c
@@ -21,6 +21,7 @@
 #include <linux/mutex.h>
 #include <linux/of_graph.h>
 #include <linux/regmap.h>
+#include <linux/slab.h>
 #include <linux/v4l2-dv-timings.h>
 
 #include <media/v4l2-ctrls.h>