Add Torsten to UAA approvers

https://github.com/torsten-sap
see
https://github.com/cloudfoundry/uaa/graphs/contributors?from=2017-12-30&to=2018-04-01&type=c

Author: strehle

toc/working-groups/foundational-infrastructure.md

@@ -107,6 +107,8 @@ areas:
     github: swalchemist
   - name: Florian Tack
     github: tack-sap
+  - name: Torsten Luh
+    github: torsten-sap
   reviewers:
   - name: Alicia Yingling
     github: Tallicia