I don’t believe there’s one included in the DSP library, but writing your own should be trivial. Just create a secondary buffer that gets allocated to the proper buffer size in your prepare method and make a copy to it as the first step in your process method, then apply a gain to both streams based on the normalized position of the dry/wet knob, and finally mix them together into the output buffer using addition.
Here’s an implementation I did for an old project using the dsp:: API. The project as a whole is GPLv3, but this class is self contained and simple feel free to take it and modify it/use it as you wish: