Update to v3.18.21 kernel headers.

Change-Id: I4ade68d62981c7f6f361750cc811e8d84be9d5b9
diff --git a/libc/kernel/uapi/linux/version.h b/libc/kernel/uapi/linux/version.h
index c927720..ddd92c3 100644
--- a/libc/kernel/uapi/linux/version.h
+++ b/libc/kernel/uapi/linux/version.h
@@ -16,5 +16,5 @@
  ***
  ****************************************************************************
  ****************************************************************************/
-#define LINUX_VERSION_CODE 201234
+#define LINUX_VERSION_CODE 201237
 #define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))