test: repai the custom_derivative test

The test imports the C library for the math library.  Add a case for
Windows.

Author: compnerd

test/AutoDiff/validation-test/custom_derivatives.swift

@@ -4,6 +4,8 @@
 import StdlibUnittest
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 import Darwin.C
+#elseif os(Windows)
+import ucrt
 #else
 import Glibc
 #endif