Add Rat excludes for go.sum

[freakyzoidberg]

Exclude go.sum for License Header check as being generated it doesn't contain any

❯ mvn apache-rat:check
[INFO] Scanning for projects...
[INFO]
[INFO] -------< org.apache.datasketches:datasketches-characterization >--------
[INFO] Building datasketches-characterization 1.0.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- apache-rat:0.13:check (default-cli) @ datasketches-characterization ---
[INFO] Enabled default license matchers.
[INFO] Will parse SCM ignores for exclusions...
[INFO] Parsing exclusions from /Users/pierre.lacave/go/src/github.com/apache/datasketches-characterization/.gitignore
[INFO] Finished adding exclusions from SCM ignore files.
[INFO] 111 implicit excludes (use -debug for more details).
[INFO] 7 explicit excludes (use -debug for more details).
[INFO] 529 resources included (use -debug for more details)
[INFO] Rat check: Summary over all files. Unapproved: 0, unknown: 0, generated: 0, approved: 313 licenses.
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.683 s
[INFO] Finished at: 2024-04-21T16:01:48-04:00
[INFO] ------------------------------------------------------------------------