Maven:While creating shaded jar, getting warning -The POM for org.knowhowlab.osgi:sigar:jar:1.6.5_01 is invalid

Question

I am creating a shaded jar with Maven shade plugin:

<plugin>                                
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-shade-plugin</artifactId>
                <version>3.1.0</version>
                <executions>
                    <execution>                 
                        <phase>package</phase>
                        <goals>
                            <goal>shade</goal>
                        </goals>
                        <configuration>
                            <source>1.7</source>
                            <target>1.7</target>
                            <filters>                               
                                <filter>
                                    <artifact>*:*</artifact>
                                    <excludes>
                                        <exclude>META-INF/*.SF</exclude>
                                        <exclude>META-INF/*.DSA</exclude>
                                        <exclude>META-INF/*.RSA</exclude>
                                    </excludes>
                                </filter>
                            </filters>
                        </configuration>                                                                        
                    </execution>                                        
                </executions>
            </plugin>

The shade plugin extracts the classes from all jars and pack them in a single jar but I get following warning for 1 jar:

[WARNING] The POM for org.knowhowlab.osgi:sigar:jar:1.6.5_01 is invalid, transitive dependencies (if any) will not be available

and the sigar jar gets added as a jar,unlike others, which I was not expecting.

The sigar jar is my pom is added like this:

<dependency>
        <groupId>org.knowhowlab.osgi</groupId>
        <artifactId>sigar</artifactId>
        <version>1.6.5_01</version>
    </dependency>

One difference I see in this sigar jar is that it contains files such as .so, .dll.

How can I make this jar get added like others in shaded jar?