Adapt packaging to make possible to write an external egl platform plugin 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>
Merge pull request #74 from Tofee/work

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