release: Update package.xml version to 5.8.1

package.xml

 <?xml version="1.0"?>
  <package format="3">
  <name>gepetto-viewer-corba</name>
- <version>5.8.0</version>
+ <version>5.8.1</version>
  <description>Corba server for gepetto-viewer </description>
 
  <maintainer email="hpp@laas.fr">Joseph Mirabel</maintainer>