cmake: set default CMAKE_BUILD_TYPE to Release
cmake: set default CMAKE_BUILD_TYPE to Release

this matches configure

Change-Id: I95f6f073b9dba1ae2c2b48b6c89a762277777dfe

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