Update to v4.15 kernel headers.

Test: Compiles, boots bullhead/hikey960.
Change-Id: I118beb8b6cac0881b1270f9bf6981959297a41a8
diff --git a/libc/kernel/uapi/linux/serial_reg.h b/libc/kernel/uapi/linux/serial_reg.h
index 95c9956..b6648f8 100644
--- a/libc/kernel/uapi/linux/serial_reg.h
+++ b/libc/kernel/uapi/linux/serial_reg.h
@@ -113,6 +113,7 @@
 #define UART_SCR 7
 #define UART_DLL 0
 #define UART_DLM 1
+#define UART_DIV_MAX 0xFFFF
 #define UART_EFR 2
 #define UART_XR_EFR 9
 #define UART_EFR_CTS 0x80