znc: link to libstdc++ to fix performance issues on channel join

ZNC uses a map to cache channel memebers on join. Unfortunately uclibc++
uses a naive linked-list implementation with horrible performance, which
can cause joins to take a few minutes on popular channels.

Therefore Switch to libstdc++ which uses a much more performant hashmap.
ZNC by itself is already rather large, so the added footprint of
libstd++ vs uclibc++ does not matter that much anyway.

Should fix #11778.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>