rtorrent: missing LDFLAG

libtorrent requires zlib to be linked

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

net/rtorrent/Makefile

  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 \