BQ should set this in its config, and does

Marlin/src/inc/Conditionals_LCD.h

@@ -62,7 +62,6 @@
   #elif ENABLED(BQ_LCD_SMART_CONTROLLER)
 
     #define REPRAP_DISCOUNT_FULL_GRAPHIC_SMART_CONTROLLER
-    #define LONG_FILENAME_HOST_SUPPORT
 
   #elif ENABLED(miniVIKI) || ENABLED(VIKI2) || ENABLED(ELB_FULL_GRAPHIC_CONTROLLER) || ENABLED(AZSMZ_12864)