rtorrent: missing LDFLAG

libtorrent requires zlib to be linked

Signed-off-by: John Crispin <blogic@openwrt.org>

net/rtorrent/Makefile

@@ -64,7 +64,7 @@ $(call Package/rtorrent/Default/description)
  This package is built with xmlrpc support
 endef
 
-TARGET_LDFLAGS += -lpthread -Wl,-rpath-link=$(STAGING_DIR)/usr/lib
+TARGET_LDFLAGS += -lz -lpthread -Wl,-rpath-link=$(STAGING_DIR)/usr/lib
 
 CONFIGURE_ARGS+= \
 	--enable-shared \