libteeacl: add pkgconfig file: teeacl.pc

Add a pkgconfig file to libteeacl, so that the library and its headers
can be found from wherever they have been installed.

Signed-off-by: Eero Aaltonen <eero.aaltonen@vaisala.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>