Go to the Settings menu, Gameplay tab, and change the "Additive Camera Motions" option. There are three, which is "Off" (no head bobbing at all), "Full" (obviously full head bobbing), and "Reduced" (a middle-ground).
I don't know why CDPR chose to name it this way instead of a simple "head bobbing" or something, but there ya go.
I don't know why CDPR chose to name it this way instead of a simple "head bobbing" or something, but there ya go.