The Protein Kinase C Inhibitor Go6976 [12-(2-Cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole] Potentiates Agonist-Induced Mitogen-Activated Protein Kinase Activation through Tyrosine Phosphorylation of the Epidermal Growth Factor Receptor