Appveyor: whitelist 3.15 branch

- DO NOT UPMERGE TO 7.0

The AppVeyor epics-base-3 project is supposed to pull the .appveyor.yml from this branch to only build commits from 3.15.

.appveyor.yml

@@ -1,6 +1,12 @@
 # .appveyor.yml for use with EPICS Base ci-scripts
 # (see: https://github.com/epics-base/ci-scripts)
 
+# branches to build
+branches:
+  # whitelist
+  only:
+    - 3.15
+
 # Version format
 version: base-{branch}-{build}