Support compilation under Swift 5.1.

Author: Simon Pilkington

.gitignore

@@ -1,4 +1,5 @@
 .DS_Store
 /.build
 /Packages
-*.xcodeproj
+.swiftpm/
+*.xcodeproj

Sources/XMLCoding/XMLStackParser.swift

@@ -8,6 +8,10 @@
 
 import Foundation
 
+#if canImport(FoundationXML)
+import FoundationXML
+#endif
+
 //===----------------------------------------------------------------------===//
 // Data Representation
 //===----------------------------------------------------------------------===//