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

Even if the code currently compiles without this, it's still better to
explicitly include the header we need.

tests/strings/strings.cpp

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