Make wx/dirdlg.h safe to include as first wx header

Include wx/defs.h to get wxUSE_DIRDLG value from it, otherwise it could be
undefined, and hence evaluate to 0 (unfortunately without even a warning with
some compilers), if this header was the first wx header to be included.

(this is a backport of 5948602f4f from master)

include/wx/dirdlg.h

@@ -11,6 +11,8 @@
 #ifndef _WX_DIRDLG_H_BASE_
 #define _WX_DIRDLG_H_BASE_
 
+#include "wx/defs.h"
+
 #if wxUSE_DIRDLG
 
 #include "wx/dialog.h"