zyxel-bootconfig: add CPPFLAGS

Add the OpenWrt CPPFLAGS to the CFLAGS. zyxel-bootconfig does not
support CPPFLAGS. This fixes fortify sources support.

Link: https://github.com/openwrt/openwrt/pull/22056
Signed-off-by: Hauke Mehrtens <[email protected]>

package/utils/zyxel-bootconfig/Makefile

@@ -26,7 +26,7 @@ endef
 define Build/Compile
 	$(MAKE) -C $(PKG_BUILD_DIR) \
 		CC="$(TARGET_CC)" \
-		CFLAGS="$(TARGET_CFLAGS) -Wall" \
+		CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS) -Wall" \
 		LDFLAGS="$(TARGET_LDFLAGS)"
 endef