kbuild: Fix Makefile.asm-generic for um

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>

scripts/Makefile.asm-generic

@@ -6,7 +6,7 @@
 # a small wrapper file in $(obj) (arch/$(SRCARCH)/include/generated/asm)
 
 kbuild-file := $(srctree)/arch/$(SRCARCH)/include/asm/Kbuild
-include $(kbuild-file)
+-include $(kbuild-file)
 
 include scripts/Kbuild.include