Windows: add wcsnlen

(backport #4721)

(cherry picked from commit 11f939a)

Author: joboet

libc-test/semver/windows.txt

@@ -348,6 +348,7 @@ utimbuf
 wchar_t
 wchmod
 wcslen
+wcsnlen
 wcstombs
 wexecl
 wexecle

src/windows/mod.rs

@@ -379,6 +379,7 @@ extern "C" {
     pub fn strtok(s: *mut c_char, t: *const c_char) -> *mut c_char;
     pub fn strxfrm(s: *mut c_char, ct: *const c_char, n: size_t) -> size_t;
     pub fn wcslen(buf: *const wchar_t) -> size_t;
+    pub fn wcsnlen(str: *const wchar_t, numberOfElements: size_t) -> size_t;
     pub fn wcstombs(dest: *mut c_char, src: *const wchar_t, n: size_t) -> size_t;
 
     pub fn memchr(cx: *const c_void, c: c_int, n: size_t) -> *mut c_void;