Ensure we use the correct reduced onelua.c file.

cvise puts the candidate reduced file in a temp dir and then it's up to
you to test it. We were copying in the source tree, but without
installing the candidate onelua.c file, so this wasn't doing what we
thought it was.

creduce/cvise.example.sh

@@ -18,7 +18,12 @@ if ! command -v yk-config 2>&1 ; then
     exit 1
 fi
 
+# Rebuild with the reduced onelua.c in-place.
+#
+# Note that it's not enough to just copy onelua.c and `Makefile` in. The whole
+# `src` dir is needed.
 cp -r "${YKLUA_HOME}/creduce/src" ./
+cp onelua.c src/
 cd ./src
 
 make onelua