NSDate: tweak whitespace (#368)

Align the `#else` with the `#if` and `#endif`.  NFC.

Author: compnerd

Foundation/NSDate.swift

@@ -11,7 +11,7 @@ import CoreFoundation
 
 #if os(OSX) || os(iOS)
     import Darwin
-    #elseif os(Linux)
+#elseif os(Linux)
     import Glibc
 #endif