doc/filters: Added line to the af_bs2b filter docs mentioning --enable-libbs2b

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/filters.texi

@@ -1911,6 +1911,9 @@ available are filtered.
 Bauer stereo to binaural transformation, which improves headphone listening of
 stereo audio records.
 
+To enable compilation of this filter you need to configure FFmpeg with
+@code{--enable-libbs2b}.
+
 It accepts the following parameters:
 @table @option