nsd: fix build_varaint usage

without this only 1 of n variants builds, all others fail

Signed-off-by: John Crispin <blogic@openwrt.org>

net/nsd/Makefile

@@ -22,6 +22,8 @@ PKG_LICENSE_FILES:=LICENSE
 
 PKG_MAINTAINER:=Vasilis Tsiligiannis <acinonyx@openwrt.gr>
 
+PKG_BUILD_DIR:=$(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_NAME)-$(PKG_VERSION)
+
 include $(INCLUDE_DIR)/package.mk
 
 define Package/nsd/default