Complete the package.xml license information

[j-rivero]

The package.xml hosted in this repository contains a minimal definition of what the Drake license is by defining it to BSD.

The use of format 3 in the package.xml of this repository allow use to declare a more accurate version of the licenses used by Drake, official spec of the XML tag is at: https://ros.org/reps/rep-0149.html#license-multiple-but-at-least-one.

Name of license for this package, e.g. BSD, GPL, LGPL. In order to assist machine readability, only include the license name in this tag. For multiple licenses multiple separate tags must be used. A package will have multiple licenses if different source files have different licenses. Every license occurring in the source files should have a corresponding tag. For any explanatory text about licensing caveats, please use the tag.