Servo changes for updated Gecko theme constants in bug 1367577.

This is a little bit odd, because I didn't want to do all the work to
convert the theme constants to an enum class (although that would be
nice to do in the future).  So this makes them a regular enum, but still
uses gecko_constant_prefix rather than gecko_enum_prefix because that
makes it possible to add the NS_THEME_ prefix that's still present.

components/style/properties/longhand/box.mako.rs

@@ -2340,7 +2340,7 @@ ${helpers.single_keyword("-moz-appearance",
                             -moz-window-titlebar-maximized
                          """,
                          gecko_ffi_name="mAppearance",
-                         gecko_constant_prefix="NS_THEME",
+                         gecko_constant_prefix="ThemeWidgetType_NS_THEME",
                          products="gecko",
                          spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-appearance)",
                          animation_value_type="none")}