samples: Add gradle build in workers sample

samples/workers/build.gradle

@@ -0,0 +1,5 @@
+apply plugin: 'konan'
+
+konanArtifacts {
+    program('Workers')
+}

samples/workers/build.sh

@@ -23,7 +23,7 @@ COMPILER_ARGS=${!var} # add -opt for an optimized build.
 
 mkdir -p $DIR/build/bin/
 
-konanc $COMPILER_ARGS -target $TARGET $DIR/Workers.kt \
+konanc $COMPILER_ARGS -target $TARGET $DIR/src/main/kotlin/Workers.kt \
        -o $DIR/build/bin/Workers || exit 1
 
 echo "Artifact could be found at $DIR/build/bin/Workers.$SUFFIX"