babl: replace a fprintf (stderr, ...) with babl_log().

babl: replace a fprintf (stderr, ...) with babl_log().

It will make it visible in the logcat, for instance in Android Studio.
As a general rule, we should use babl_log() for anything which we
consider as "logs", unless we want to direct them to a given output for
sure (for instance in the case of CLI tools where we want the output to
be on standard I/O in terminal, fprintf() still is the best choice).