fbdev: Do not include <linux/slab.h> in header

Forward declare struct page and remove the include statement.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240219093941.3684-8-tzimmermann@suse.de

include/linux/fb.h

@@ -11,7 +11,6 @@
 #include <linux/init.h>
 #include <linux/workqueue.h>
 #include <linux/list.h>
-#include <linux/slab.h>
 
 #include <asm/fb.h>
 
@@ -22,6 +21,7 @@ struct device;
 struct file;
 struct inode;
 struct notifier_block;
+struct page;
 struct videomode;
 struct device_node;