validation-test: make stdlib.string compile on Win32

The C library functions used here do not appear without the C runtime
being imported (ucrt or MSVCRT).  Adjust the test as such.  This exposes
a stack corruption in the test which needs to be resolved.

Author: compnerd

validation-test/stdlib/String.swift

@@ -20,6 +20,10 @@ import NSSlowString
 import Foundation  // For NSRange
 #endif
 
+#if os(Windows)
+import ucrt
+#endif
+
 extension Collection {
   internal func index(_nth n: Int) -> Index {
     precondition(n >= 0)