Fix cmake -DENABLE_EXAMPLES=0 error.
Fix cmake -DENABLE_EXAMPLES=0 error.

This cmake error was introduced in commit
bdf8bf9f9c165de726d9383ed123bb11a6eeddf9:
https://aomedia-review.googlesource.com/c/aom/+/70787

Add aom_encoder_stats to AOM_LIB_TARGETS in only one place, if
ENABLE_EXAMPLES is nonzero.

Change-Id: Ia29f9df73be7d78c0a753c04845a080556895a75

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