opus-tools: disable flac when it is not selected

opus-tools expects flac since the 0.1.7 bump unless otherwise told so.
Disable it when it's not selected. Fixes:
http://autobuild.buildroot.net/results/c8c/c8c1a682336c150ee401f76b173944611b96bd4e/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>