Manpages of src:rtfilter in Debian testing

• rtf_create_bandpass_analytic(3)
• rtf_create_butterworth(3)
• rtf_create_chebychev(3)
• rtf_create_downsampler(3)
• rtf_create_filter(3)
• rtf_create_fir_bandpass(3)
• rtf_create_fir_highpass(3)
• rtf_create_fir_lowpass(3)
• rtf_destroy_filter(3)
• rtf_filter(3)
• rtf_get_type(3)
• rtf_get_version(3)
• rtf_init_filter(3)