ucode-mod-bpf: add CPPFLAGS

Forward the OpenWrt CPPFLAGS to the compile process. This fixes fortify
sources support.

Link: openwrt#22056
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit c3f2a09)

Author: hauke

package/utils/ucode-mod-bpf/Makefile

@@ -32,7 +32,7 @@ define Build/Configure
 endef
 
 define Build/Compile
-	$(TARGET_CC) $(TARGET_CFLAGS) $(TARGET_LDFLAGS) $(FPIC) \
+	$(TARGET_CC) $(TARGET_CPPFLAGS) $(TARGET_CFLAGS) $(TARGET_LDFLAGS) $(FPIC) \
 		-Wall -ffunction-sections -Wl,--gc-sections -shared -Wl,--no-as-needed -lbpf \
 		-o $(PKG_BUILD_DIR)/bpf.so $(PKG_BUILD_DIR)/bpf.c
 endef