kbuild: fix O=... build of um

We used include/asm-$ARCH/system.h to check if
we should create a symlink in include2 directory with
make O=... builds.
But um does not have such a file thus build filed.

Let's try anohter filename:
$ ls -d include/asm-* | wc -l
21
$ ls -d include/asm-*/errno.h | wc -l
21

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Jeff Dike <jdike@addtoit.com>

Makefile

@@ -930,7 +930,7 @@ ifneq ($(KBUILD_SRC),)
 		/bin/false; \
 	fi;
 	$(Q)if [ ! -d include2 ]; then mkdir -p include2; fi;
-	$(Q)if [ -e $(srctree)/include/asm-$(SRCARCH)/system.h ]; then  \
+	$(Q)if [ -e $(srctree)/include/asm-$(SRCARCH)/errno.h ]; then  \
 	    ln -fsn $(srctree)/include/asm-$(SRCARCH) include2/asm;     \
 	    fi
 endif