Update to v4.15 kernel headers.

Test: Compiles, boots bullhead/hikey960.
Change-Id: I118beb8b6cac0881b1270f9bf6981959297a41a8
diff --git a/libc/kernel/uapi/linux/magic.h b/libc/kernel/uapi/linux/magic.h
index 6e0ddf7..68b4c65 100644
--- a/libc/kernel/uapi/linux/magic.h
+++ b/libc/kernel/uapi/linux/magic.h
@@ -61,6 +61,7 @@
 #define OPENPROM_SUPER_MAGIC 0x9fa1
 #define QNX4_SUPER_MAGIC 0x002f
 #define QNX6_SUPER_MAGIC 0x68191122
+#define AFS_FS_MAGIC 0x6B414653
 #define REISERFS_SUPER_MAGIC 0x52654973
 #define REISERFS_SUPER_MAGIC_STRING "ReIsErFs"
 #define REISER2FS_SUPER_MAGIC_STRING "ReIsEr2Fs"