Make wx/valnum.h self-contained by including wx/textentry.h

This header uses (i.e. forward declaration wouldn't be sufficient)
wxTextEntry, so it must include the header declaring it.

(cherry picked from commit 9db6f6ca1a)

include/wx/valnum.h

@@ -15,6 +15,7 @@
 
 #if wxUSE_VALIDATORS
 
+#include "wx/textentry.h"
 #include "wx/validate.h"
 
 #include <limits>