um: Stop abusing __KERNEL__

Currently UML is abusing __KERNEL__ to distinguish between
kernel and host code (os-Linux). It is better to use a custom
define such that existing users of __KERNEL__ don't get confused.

Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/arch/um/include/shared/user.h b/arch/um/include/shared/user.h
index cef0685..4cff19f 100644
--- a/arch/um/include/shared/user.h
+++ b/arch/um/include/shared/user.h
@@ -17,7 +17,7 @@
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
 
 /* This is to get size_t */
-#ifdef __KERNEL__
+#ifndef __UM_HOST__
 #include <linux/types.h>
 #else
 #include <stddef.h>