cogl-path: Make cogl_path_arc_rel static

cogl-path: Make cogl_path_arc_rel static

cogl_path_arc_rel was never in any public headers so it isn't part of
the public API. It also has a slightly inconsistent name because the
rest of the relative path functions are called cogl_path_rel_*. This
patch makes it static for now to make it more obvious that it isn't
public. The name has changed to _cogl_path_rel_arc.