kbuild: prevent lib-ksyms.o rebuilds

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reported-by: Russell King <rmk+kernel@armlinux.org.uk>
Tested-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Michal Marek <mmarek@suse.com>

scripts/Makefile.build

@@ -430,6 +430,9 @@ cmd_export_list = $(OBJDUMP) -h $< | \
 
 $(obj)/lib-ksyms.o: $(lib-target) FORCE
 	$(call if_changed,export_list)
+
+targets += $(obj)/lib-ksyms.o
+
 endif
 
 #