Update default.xml

Add swift-subprocess to the checkout set.

Author: compnerd

default.xml

@@ -41,6 +41,7 @@
   <project remote="github" name="apple/swift-numerics" path="swift-numerics" groups="notdefault" />
   <project remote="github" name="swiftlang/swift-package-manager" path="swift-package-manager" />
   <project remote="github" name="apple/swift-protobuf" path="swift-protobuf" groups="notdefault" />
+  <project remote="github" name="swiftlang/swift-subprocess" path="swift-subprocess" />
   <project remote="github" name="swiftlang/swift-syntax" path="swift-syntax" />
   <project remote="github" name="apple/swift-system" path="swift-system" revision="refs/tags/1.5.0" />
   <project remote="github" name="swiftlang/swift-testing" path="swift-testing" />