Use primitive to allow it to be resolved by @value in javadoc

Currently following is reported:
```
[ERROR] Exit code: 1 - .../src/main/java/org/codehaus/mojo/buildhelper/ReserveListenerPortMojo.java:100: error: value does not refer to a constant
[ERROR]      * If {@link #maxPortNumber} is specified, defaults to {@value #FIRST_NON_ROOT_PORT_NUMBER}.
[ERROR]                                                            ^
```

src/main/java/org/codehaus/mojo/buildhelper/ReserveListenerPortMojo.java

@@ -62,7 +62,7 @@ public class ReserveListenerPortMojo
 {
     private static final String BUILD_HELPER_RESERVED_PORTS = "BUILD_HELPER_MIN_PORT";
 
-    private static final Integer FIRST_NON_ROOT_PORT_NUMBER = 1024;
+    private static final int FIRST_NON_ROOT_PORT_NUMBER = 1024;
 
     private static final Integer MAX_PORT_NUMBER = 65535;