Add pkgconfig file for libteec

Autotools build environments that use libteec would benefit from the
addition of generated package config files so that libraries could be
found wherever they are installed.  Add a teec pkg-config generation to
support that

Signed-off-by: Neil Horman <nhorman@gmail.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>