cmake: fix -DCONFIG_GCOV=1
cmake: fix -DCONFIG_GCOV=1

the coverage flags are required at link time as well as compile time

Change-Id: Ic581f8ce747615f1fa1dc13d989d6e233e982572

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