🐛 Fix FTDI::get_utf8_char_and_inc compile (#24048)

Marlin/src/lcd/extui/ftdi_eve_touch_ui/ftdi_eve_lib/extended/unicode/unicode.cpp

     return val;
   }
 
-  utf8_char_t FTDI::get_utf8_char_and_inc(char *&c) {
+  utf8_char_t FTDI::get_utf8_char_and_inc(const char *&c) {
     utf8_char_t val = *(uint8_t*)c++;
     if ((val & 0xC0) == 0xC0)
       while ((*c & 0xC0) == 0x80)