util/WStringAPI: add wcscoll() wrapper

src/util/StringAPI.hxx

 /*
- * Copyright (C) 2010-2017 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2010-2018 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions

src/util/WStringAPI.hxx

@@ -168,6 +168,13 @@ StringIsEqualIgnoreCase(const wchar_t *a, const wchar_t *b,
 
 #endif
 
+gcc_pure gcc_nonnull_all
+static inline int
+StringCollate(const wchar_t *a, const wchar_t *b) noexcept
+{
+	return wcscoll(a, b);
+}
+
 gcc_malloc gcc_returns_nonnull gcc_nonnull_all
 static inline wchar_t *
 DuplicateString(const wchar_t *p) noexcept