squid: disable netfilter for musl toolchains

It has been broken for several releases, fixes:
http://autobuild.buildroot.net/results/d6c/d6cf725e88bc0a4afd0b7e1ae58a8e5a649ae6c3/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/squid/squid.mk

@@ -25,7 +25,7 @@ SQUID_CONF_ENV = \
 	BUILDCXXFLAGS="$(HOST_CXXFLAGS)"
 SQUID_CONF_OPTS = \
 	--enable-async-io=8 \
-	--enable-linux-netfilter \
+	$(if $(BR2_TOOLCHAIN_USES_MUSL),--disable-linux-netfilter,--enable-linux-netfilter) \
 	--enable-removal-policies="lru,heap" \
 	--with-filedescriptors=1024 \
 	--disable-ident-lookups \