JSON: enable coverage report for encoding/decoding code

tools/travis/travis_linux_script.sh

@@ -414,6 +414,7 @@ if [ "$CC" != "tcc" ]; then
         -DUA_ENABLE_DISCOVERY=ON \
         -DUA_ENABLE_DISCOVERY_MULTICAST=ON \
         -DUA_ENABLE_ENCRYPTION=ON \
+        -DUA_ENABLE_JSON_ENCODING=ON \
         -DUA_ENABLE_PUBSUB=ON \
         -DUA_ENABLE_PUBSUB_DELTAFRAMES=ON \
         -DUA_ENABLE_PUBSUB_INFORMATIONMODEL=ON \