fix the bind subdir parallel builds

https://bugs.gentoo.org/380717

--- a/bind/lib/export/isc/Makefile.in
+++ b/bind/lib/export/isc/Makefile.in
@@ -114,6 +114,7 @@
 		-DLIBAGE=${LIBAGE} \
 		-c ${srcdir}/version.c
 
+${OBJS}: | subdirs
 libisc.@SA@: ${OBJS}
 	${AR} ${ARFLAGS} $@ ${OBJS}
 	${RANLIB} $@
