fix main method definition

src/main/java/org/jastadd/ros2rag/compiler/Compiler.java

@@ -65,12 +65,12 @@ public class Compiler {
   }
 
 
-  public static int main(String[] args) {
+  public static void main(String[] args) {
     try {
-      return new Compiler().run(args);
+      new Compiler().run(args);
     } catch (CommandLineException | CompilerException e) {
       System.err.println(e.getMessage());
-      return 1;
+      System.exit(-1);
     }
   }