egl: Add EGL_RESOURCE_BUSY_EXT error

This change adds both the error code and description for
EGL_RESOURCE_BUSY_EXT, recently added to the EGL_EXT_stream_acquire_mode
extension:

  https://github.com/aritger/eglstreams-kms-example/blob/master/proposed-extensions/EGL_EXT_stream_acquire_mode.txt

https://bugzilla.gnome.org/show_bug.cgi?id=779112