sandbox/fft_r2r_test: removing pre-processor warning
sandbox/fft_r2r_test: removing pre-processor warning

JayKickliter/liquid-dsp,jgaeddert/liquid-dsp,JayKickliter/liquid-dsp,jgaeddert/liquid-dsp,wangning223/liquid-dsp,wangning223/liquid-dsp,wangning223/liquid-dsp,JayKickliter/liquid-dsp,JayKickliter/liquid-dsp,jgaeddert/liquid-dsp,wangning223/liquid-dsp,jgaeddert/liquid-dsp,jgaeddert/liquid-dsp,JayKickliter/liquid-dsp,wangning223/liquid-dsp