Resolve deprecation

Sources/Build/BuildDescription/MixedTargetBuildDescription.swift

@@ -80,7 +80,7 @@ public final class MixedTargetBuildDescription {
             throw InternalError("underlying target type mismatch \(target)")
         }
 
-        guard buildParameters.triple.isDarwin() else {
+        guard buildParameters.targetTriple.isDarwin() else {
             throw StringError("Targets with mixed language sources are only " +
                 "supported on Apple platforms.")
         }