Adapt packaging to make possible to write an external egl platform plugi...

...n for libhybris

* add the files ws.h and eglplatformcommon.h in the {includedir}/hybris/eglplatformcommon directory
 * create a hybris-egl-platform pkg-config file description

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>