Do nothing if arch/$(SRCARCH)/include/asm/Kbuild does not exist, which
is the case of um.
Reported-by: NRandy Dunlap <rdunlap@xenotime.net>
Signed-off-by: NMichal Marek <mmarek@suse.cz>
Acked-by: NRandy Dunlap <rdunlap@xenotime.net>
Acked-by: NSam Ravnborg <sam@ravnborg.org>