tcp_wrappers: fix compilation warnings

Set `Wall` and address sources of reported warnings.

Signed-off-by: Tom Sullivan <tom@msbit.com.au>

libs/tcp_wrappers/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=tcp_wrappers
 PKG_VERSION:=7.6
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)_$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=ftp://ftp.porcupine.org/pub/security
@@ -30,7 +30,7 @@ define Package/libwrap
   MAINTAINER:=Peter Wagner <tripolar@gmx.at>
 endef
 
-TARGET_CFLAGS += $(FPIC)
+TARGET_CFLAGS += $(FPIC) -Wall
 
 ifeq ($(CONFIG_USE_MUSL),)
 TARGET_EXTRA_LIBS:=LIBS=-lnsl