Update to v4.15 kernel headers.

Test: Compiles, boots bullhead/hikey960.
Change-Id: I118beb8b6cac0881b1270f9bf6981959297a41a8
diff --git a/libc/kernel/uapi/linux/prctl.h b/libc/kernel/uapi/linux/prctl.h
index a30fe17..55098f5 100644
--- a/libc/kernel/uapi/linux/prctl.h
+++ b/libc/kernel/uapi/linux/prctl.h
@@ -129,4 +129,9 @@
 #define PR_CAP_AMBIENT_RAISE 2
 #define PR_CAP_AMBIENT_LOWER 3
 #define PR_CAP_AMBIENT_CLEAR_ALL 4
+#define PR_SVE_SET_VL 50
+#define PR_SVE_SET_VL_ONEXEC (1 << 18)
+#define PR_SVE_GET_VL 51
+#define PR_SVE_VL_LEN_MASK 0xffff
+#define PR_SVE_VL_INHERIT (1 << 17)
 #endif