WebFFTFrame: Fix unused parameter warnings

This commit solves the following warnings:
WebFFTFrame.h:36:18: warning: unused parameter 'data' [-Wunused-parameter]
WebFFTFrame.h:37:18: warning: unused parameter 'data' [-Wunused-parameter]
WebFFTFrame.h:38:18: warning: unused parameter 'frame' [-Wunused-parameter]

Reviewed by: Hugo Lima