Fix build problem with MSVS 2005 in wx/msw/uxtheme.h

Avoid conflict with the incomplete MENUPARTS enum defined in the SDK
header included with this compiler.

Closes #18217.

docs/changes.txt

@@ -136,6 +136,7 @@ wxMSW:
 - Fix build in ANSI (non-Unicode) mode.
 - Improve wxNotebook themed background drawing (Arrigo Marchiori).
 - Send wxEVT_WEBVIEW_NAVIGATING when redirecting (Josue Andrade Gomes).
+- Fix build with MSVS 2005 broken in 3.1.1.
 
 wxOSX:
 

include/wx/msw/uxtheme.h

@@ -112,7 +112,10 @@ enum BORDERSTATES {
     CBB_DISABLED = 4,
 };
 
-enum MENUPARTS
+// The MENUPARTS enum is defined in MSVS 2005 SDK, even though it doesn't have
+// vssym32.h, but it doesn't define the constants we use, so still define them,
+// but make the enum unnamed for compatibility.
+enum /* MENUPARTS -- FIXME-VC8: uncomment this when support for it is dropped */
 {
     MENU_MENUITEM_TMSCHEMA = 1,
     MENU_SEPARATOR_TMSCHEMA = 6,