Merge pull request #9086 from fiveangle/bf2-hasmesh_on_g29s3

[2.0.x] Set has_mesh if user issues G9 S3 with MBL

Marlin/src/gcode/bedlevel/mbl/G29.cpp

         SERIAL_CHAR('Z'); echo_not_entered();
         return;
       }
+
+      mbl.has_mesh = true; // set since user manually entered a mesh point
+
       break;
 
     case MeshSetZOffset: