Make wx/stackwalk.h self-sufficient

Don't rely on wx/string.h being already included, but include it explicitly
ourselves to fix compilation error when wx/stackwalk.h is the first wx header
to be included.

include/wx/stackwalk.h

@@ -15,6 +15,8 @@
 
 #if wxUSE_STACKWALKER
 
+#include "wx/string.h"
+
 class WXDLLIMPEXP_FWD_BASE wxStackFrame;
 
 #define wxSTACKWALKER_MAX_DEPTH       (200)