firewall: compile with LTO enabled

Reduces .ipk size on MIPS from 41.6k to 41.1k

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/network/config/firewall/Makefile

@@ -40,8 +40,8 @@ define Package/firewall/conffiles
 /etc/firewall.user
 endef
 
-TARGET_CFLAGS += -ffunction-sections -fdata-sections
-TARGET_LDFLAGS += -Wl,--gc-sections
+TARGET_CFLAGS += -ffunction-sections -fdata-sections -flto
+TARGET_LDFLAGS += -Wl,--gc-sections -flto
 CMAKE_OPTIONS += $(if $(CONFIG_IPV6),,-DDISABLE_IPV6=1)
 
 define Package/firewall/install