Update to v5.0 kernel headers.

Test: Builds and boots on taimen.
Change-Id: I13843bf1ab30ea89a50852adc88f2cba8401bded
diff --git a/libc/kernel/uapi/linux/fb.h b/libc/kernel/uapi/linux/fb.h
index 9f6b139..38da042 100644
--- a/libc/kernel/uapi/linux/fb.h
+++ b/libc/kernel/uapi/linux/fb.h
@@ -320,4 +320,6 @@
   struct fbcurpos hot;
   struct fb_image image;
 };
+#define FB_BACKLIGHT_LEVELS 128
+#define FB_BACKLIGHT_MAX 0xFF
 #endif