Update to v4.15 kernel headers.

Test: Compiles, boots bullhead/hikey960.
Change-Id: I118beb8b6cac0881b1270f9bf6981959297a41a8
diff --git a/libc/kernel/uapi/asm-generic/mman.h b/libc/kernel/uapi/asm-generic/mman.h
index 0452237..84caab7 100644
--- a/libc/kernel/uapi/asm-generic/mman.h
+++ b/libc/kernel/uapi/asm-generic/mman.h
@@ -28,6 +28,7 @@
 #define MAP_NONBLOCK 0x10000
 #define MAP_STACK 0x20000
 #define MAP_HUGETLB 0x40000
+#define MAP_SYNC 0x80000
 #define MCL_CURRENT 1
 #define MCL_FUTURE 2
 #define MCL_ONFAULT 4