arptables: add CPPFLAGS

Add the OpenWrt CPPFLAGS to the CFLAGS. arptables 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/network/utils/arptables/Makefile

@@ -31,7 +31,7 @@ define Package/arptables-legacy
 endef
 
 MAKE_FLAGS += \
-	COPT_FLAGS="$(TARGET_CFLAGS) -D__OPTIMIZE__=1" \
+	COPT_FLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS) -D__OPTIMIZE__=1" \
 	KERNEL_DIR="$(LINUX_DIR)"
 
 define Package/arptables-legacy/install