Foundation: fix a couple of compile errors

FoundationErrors does not depend on NSSwiftRuntime and thus does not
transitively get the WinSDK import when not building in whole program
mode.  Add an import to get a definition of `DWORD`.

Remoe the `private(set)` from the computed `processIdentifier` property
which is already readonly due to the definition of the computed
property.

Author: compnerd

Foundation/FoundationErrors.swift

@@ -7,6 +7,10 @@
 // See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 
+#if os(Windows)
+import WinSDK
+#endif
+
 /// File-system operation attempted on non-existent file.
 public var NSFileNoSuchFileError: Int                        { return CocoaError.Code.fileNoSuchFile.rawValue }
 

Foundation/Process.swift

@@ -777,7 +777,7 @@ open class Process: NSObject {
     // status
 #if os(Windows)
     open private(set) var processHandle: HANDLE = INVALID_HANDLE_VALUE
-    open private(set) var processIdentifier: Int32 {
+    open var processIdentifier: Int32 {
       return Int32(GetProcessId(processHandle))
     }
     open private(set) var isRunning: Bool = false