Fix #122491: DOF pick in context menu grayed out

Caused by 7f8d4df410
Changes in poll function had excluded the case of context menu button.
Fix this by returning true if operator identifier is `UI_OT_eyedropper_depth`

Pull Request: https://projects.blender.org/blender/blender/pulls/122532

source/blender/editors/interface/eyedroppers/eyedropper_depth.cc

@@ -442,6 +442,11 @@ static bool depthdropper_poll(bContext *C)
     if (but->icon == ICON_EYEDROPPER) {
       return true;
     }
+    /* Context menu button. */
+    if (but->optype && STREQ(but->optype->idname, "UI_OT_eyedropper_depth")) {
+      return true;
+    }
+
     if ((but->type == UI_BTYPE_NUM) && (prop != nullptr) &&
         (RNA_property_type(prop) == PROP_FLOAT) &&
         (RNA_property_subtype(prop) & PROP_UNIT_LENGTH) &&