mesa: Unify glEdgeFlagPointer data type.

Use GL_UNSIGNED_BYTE as initialization data type
for the edge flag vertex attribute array. The same datatype
is used in the glEdgeFlagPointer function when setting the
array pointer.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>