A Study Comparing Palbociclib Plus Cetuximab Versus Cetuximab for the Treatment of Head and Neck Cancer. A Randomized, Multicenter, Double-Blind Phase 2 Study of Palbociclib Plus Cetuximab Versus Cetuximab for the Treatment of Human Papillomavirus-Negative, Cetuximab-Naïve Participants With Recurrent/Metastatic Squamous Cell Carcinoma of the Head and Neck After Failure of One Prior Platinum-Containing Chemotherapy Regimen.