Fixes #20905 error: 'G28_STR' was not declared in this scope

[ellensp]

Description

enabling ANYCUBIC_LCD_I3MEGA results in a compile error

Marlin/src/lcd/extui/lib/anycubic_i3mega/anycubic_i3mega_lcd.cpp: In member function 'void AnycubicTFTClass::GetCommandFromTFT()':
Marlin/src/lcd/extui/lib/anycubic_i3mega/anycubic_i3mega_lcd.cpp:768:41: error: 'G28_STR' was not declared in this scope
                 ExtUI::injectCommands_P(G28_STR);

This adds external reference to G28_STR so it can be resolved.

Requirements

Current bugfix and enable ANYCUBIC_LCD_I3MEGA

Benefits

Compiles as expected

Related Issues

Issue #20905