Set COGL_WINSYS_FEATURE_BUFFER_AGE on EGL

This winsys feature flag is exposed via the deprecated
cogl_clutter_winsys_has_feature function and Clutter is curently
relying on it. Previously the EGL winsys was only setting the internal
COGL_EGL_WINSYS_FEATURE_BUFFER_AGE flag and there was no mapping to
the public flag. Therefore the feature would only be used on GLX. This
patch just adds the mapping.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 8418e98b2b1b25515a961ad1bb9f0c4770d6eb1d)