Fix build break by adding missing Import

There was a race between #1174 and #1176 which removed an import.

Author: artemcm

Sources/SwiftDriver/SwiftScan/SwiftScan.swift

@@ -18,6 +18,7 @@ import func Foundation.free
 import protocol TSCBasic.DiagnosticData
 import struct TSCBasic.AbsolutePath
 import struct TSCUtility.Version
+import struct TSCBasic.Diagnostic
 
 public enum DependencyScanningError: Error, DiagnosticData {
   case missingRequiredSymbol(String)