Fix compile error in menu_motion when enabled FT_MOTION_MENU

[marbocub]

Description

There is a compile error in menu_motion.cpp when enabled FT_MOTION_MENU. The error message is:

Marlin\src\lcd\menu\menu_motion.cpp: In lambda function:
Marlin\src\lcd\menu\menu_motion.cpp:437:7: error: 'c' is not captured
       c.active ^= true;
       ^

The error occurs at capturing the c in the lambda function due to it not using capture syntax [&].
However, when using [&], the new error occurs because it cannot be converted from <lambda()> to screenFunc_t.

This PR fixes the error by declaring the new reference c within lambda. That is the same reference as the upper c.

This code may not be optimal, for example it can change ftMotion.cfg.active directly without using the c as before #26670. But I think it's readable to use the c for consistency with other codes.

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