programs/sndfile-convert.c : Add warning for some conversions.
programs/sndfile-convert.c : Add warning for some conversions.

Specifically when converting to GSM 6.10 when the input samplerate
is not 8kHz no resampling will be done so when the file is played
it will simply sound like the original input file played at the
new sample rate.

Suggested-by: Jan Stary <8800578b51f022c8d8adb9606a8b3db4fedbdac6@stare.cz>

erikd/libsndfile,RonNovy/libsndfile,Icenowy/libsndfile,evpobr/libsndfile,RonNovy/libsndfile,audiokit/libsndfile,erikd/libsndfile,evpobr/libsndfile,syb0rg/libsndfile,libsndfile/libsndfile,syb0rg/libsndfile,libsndfile/libsndfile,Icenowy/libsndfile,evpobr/libsndfile,Icenowy/libsndfile,RonNovy/libsndfile,Icenowy/libsndfile,libsndfile/libsndfile,audiokit/libsndfile,RonNovy/libsndfile,audiokit/libsndfile,syb0rg/libsndfile,erikd/libsndfile,erikd/libsndfile,audiokit/libsndfile,syb0rg/libsndfile,erikd/libsndfile,libsndfile/libsndfile,evpobr/libsndfile,Icenowy/libsndfile,evpobr/libsndfile,libsndfile/libsndfile