libwebos-gui: add eglplatform_offscreen file to package.

The libwebos-gui compilation now produces an additional library, "eglplatform_offscreen", intended to be an egl platform for libhybris.

Nota Bene: this PR belongs to the following set of PR for webOS-ports: libwebos-gui, qt-webos-plugin, webappmanager, luna-sysmgr and meta-webos-ports.