[klibc] Makefile: Add dependencies on $(KLIBCKERNELSRC)

The error message for a missing $(KLIBCKERNELSRC) won't appear
unless it's specified as a target or something depends on it.

Make the klcc and klibc targets depend on it.

Link: https://www.zytor.com/pipermail/klibc/2019-January/004030.html
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
diff --git a/Makefile b/Makefile
index cd15149..4386135 100644
--- a/Makefile
+++ b/Makefile
@@ -112,10 +112,10 @@
 	sed -e 's/@@VERSION@@/$(VERSION)/g' < $< > $@
 
 # Build klcc - it is the first target
-klcc: $(objtree)/.config
+klcc: $(objtree)/.config $(KLIBCKERNELSRC)
 	$(Q)$(MAKE) $(klibc)=klcc
 
-klibc: $(objtree)/.config
+klibc: $(objtree)/.config $(KLIBCKERNELSRC)
 	$(Q)$(MAKE) $(klibc)=.
 
 test: klibc