UAPI: Make arch/mn10300/include/uapi/asm/setup.h non-empty

arch/mn10300/include/uapi/asm/setup.h was emitted by the UAPI disintegration
script as an empty file because the parent file had no UAPI stuff in it,
despite being marked with "header-y".

Unfortunately, the patch program deletes resultant empty files when applying a
kernel patch.

So just stick a comment in there as a placeholder.

Signed-off-by: David Howells <dhowells@redhat.com>

arch/mn10300/include/uapi/asm/setup.h

@@ -0,0 +1,4 @@
+/*
+ * There isn't anything here anymore, but the file must not be empty or patch
+ * will delete it.
+ */