Include errno.h before using errno in the string unit test

This fixes compilation after the changes of abe5da9875 (Restore value of
errno after wxString::ToInt()/Long()/etc., 2023-01-03) under some
platforms, where this header doesn't get included from elsewhere, and
it's better to explicitly include the headers we need anyhow.

See #23113.

tests/strings/strings.cpp

@@ -19,6 +19,8 @@
 
 #include "wx/private/localeset.h"
 
+#include <errno.h>
+
 // ----------------------------------------------------------------------------
 // test class
 // ----------------------------------------------------------------------------