Only build deringing code when --enable-dering.
Only build deringing code when --enable-dering.

Change-Id: Icfd4654d2fe43ab4851cd86dfedf9639cd4abab3

smarter/aom,thdav/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,shacklettbp/aom,mbebenita/aom,jmvalin/aom,GrokImageCompression/aom,thdav/aom,smarter/aom,mbebenita/aom,luctrudeau/aom,thdav/aom,jmvalin/aom,jmvalin/aom,mbebenita/aom,thdav/aom,luctrudeau/aom,luctrudeau/aom,mbebenita/aom,shacklettbp/aom,thdav/aom,smarter/aom,jmvalin/aom,mbebenita/aom,luctrudeau/aom,thdav/aom,smarter/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,jmvalin/aom,shacklettbp/aom,shacklettbp/aom,GrokImageCompression/aom,shacklettbp/aom,jmvalin/aom,shacklettbp/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,GrokImageCompression/aom,GrokImageCompression/aom