koji: small patch to fix warnings

[John Morris]

After the koji.conf 'pkgurl' attribute was enabled (sigul wants this; 
I'll submit a patch for that too, later), one of my scripts broke 
because the "Warning: the pkgurl option is obsolete" message is printed 
to stdout instead of stderr.  Here's a little patch to print the warning 
with the warn() function instead.

I realize some warnings from the cli might be best printed to stdout, 
but for ones that might appear when using a computer-readable type of 
command like 'list-* --quiet', stderr seems like the better choice.

Thanks-

John