zstring_view: Fix function specification

Friend function is noexcept but it wasn't correctly specified.

Change-Id: I5b7a2ac50498cff6e1ffc3264eaacca12d8eeb7c
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/stdplus/zstring_view.hpp b/include/stdplus/zstring_view.hpp
index a346130..79003bd 100644
--- a/include/stdplus/zstring_view.hpp
+++ b/include/stdplus/zstring_view.hpp
@@ -296,7 +296,8 @@
     {};
     constexpr basic_zstring_view(unsafe, string_view_base sv) noexcept : sv(sv)
     {}
-    friend auto detail::unsafe_zstring_view<CharT, Traits>(string_view_base sv);
+    friend constexpr auto detail::unsafe_zstring_view<CharT, Traits>(
+        string_view_base sv) noexcept;
 };
 
 template <typename CharT, typename Traits>