Cannibalism is the act or practice of humans eating the flesh or internal organs of other deceased human beings. The placenta is totally unique in that it is actually made up from both the fetus and the mother, so it belongs 100% to neither one nor the other. The placenta is a fetomaternal organ with two components: the fetal placenta (Chorion frondosum), which develops from the same blastocyst that forms the fetus, and the maternal placenta (Decidua basalis), which develops from the maternal uterine tissue.
So no, it’s not cannibalism for a mother to eat her own placenta, as it is her own organ (albeit shared with her baby) and nobody has died as a result of her getting it.