Fix CONFIG_BGSPRITE builds with CMake.
Fix CONFIG_BGSPRITE builds with CMake.

Add av1/encoder/bgsprite.[ch] to the CMake build.

BUG=aomedia:683

Change-Id: Ic90c81c724be6814a3534f2f612173ac325d4423

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom