Fix checkstyle error when running specific Maven module

Fixes:

```
Failed during checkstyle configuration: cannot initialize module Header - illegal value '.checkstyle-header' for property 'headerFile': com.puppycrawl.tools.checkstyle.api.CheckstyleException: Unable to find: .checkstyle-header
```

When running a specific module (e.g. `vulnerability-analyzer`) via `mvn quarkus:dev`.

Signed-off-by: nscuro <[email protected]>

Author: nscuro

.checkstyle.xml

@@ -6,7 +6,7 @@
     <property name="localeLanguage" value="en"/>
     <module name="Header">
         <property name="charset" value="UTF-8"/>
-        <property name="headerFile" value=".checkstyle-header"/>
+        <property name="headerFile" value="${parentBaseDir}/.checkstyle-header"/>
     </module>
     <module name="TreeWalker">
         <module name="AvoidStarImport"/>

pom.xml

@@ -507,6 +507,9 @@
           <includeTestSourceDirectory>true</includeTestSourceDirectory>
           <includeResources>false</includeResources>
           <includeTestResources>false</includeTestResources>
+          <propertyExpansion>
+            parentBaseDir=${project.parentBaseDir}
+          </propertyExpansion>
         </configuration>
         <executions>
           <execution>