Add missing libc library spec that weren't added to the ext-toolchain
script when the library were introduced in the packages libs toolchain
Makefile.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit 8cad52a267bffe384a119f3e5ae1892e8580a981)
Link: https://github.com/openwrt/openwrt/pull/15898
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>