Crystal structure of catena-poly[diaquabis(μ2-3-carboxybenzene-1,2-dicarboxylato-1:2κ2O:O′)-(μ2-1,4-bis(2-ethylbenzimidazol-1-ylmethyl)-benzene-1:1′κ2N:N′)dizinc(II)], [Zn2(C26H26N4)(C9H4O6)2(H2O)2]