Update to v4.15 kernel headers.

Test: Compiles, boots bullhead/hikey960.
Change-Id: I118beb8b6cac0881b1270f9bf6981959297a41a8
diff --git a/libc/kernel/uapi/linux/btrfs_tree.h b/libc/kernel/uapi/linux/btrfs_tree.h
index c75732e..c2fbd40 100644
--- a/libc/kernel/uapi/linux/btrfs_tree.h
+++ b/libc/kernel/uapi/linux/btrfs_tree.h
@@ -320,6 +320,7 @@
 #define BTRFS_FILE_EXTENT_INLINE 0
 #define BTRFS_FILE_EXTENT_REG 1
 #define BTRFS_FILE_EXTENT_PREALLOC 2
+#define BTRFS_FILE_EXTENT_TYPES 2
 struct btrfs_file_extent_item {
   __le64 generation;
   __le64 ram_bytes;