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

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index a58e54f..cae4509 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -234,8 +234,7 @@
 endif()
 
 # List of object and static library targets.
-set(AOM_LIB_TARGETS ${AOM_LIB_TARGETS} aom_rtcd aom_encoder_stats aom_mem
-    aom_scale aom)
+set(AOM_LIB_TARGETS ${AOM_LIB_TARGETS} aom_rtcd aom_mem aom_scale aom)
 
 # Setup dependencies.
 setup_aom_dsp_targets()