buildroot: fix installation of TA shared libraries

Fixes a mistake in the optee_os install step which prevents the
libraries from being copied into the root FS staging area.

Fixes: 91ebff72 ("buildroot: add optee_os package to copy shared libraries into the root FS")
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>