Add Cap:LONG_FILENAME (#17821)

Marlin/src/gcode/host/M115.cpp

     // AUTOREPORT_SD_STATUS (M27 extension)
     cap_line(PSTR("AUTOREPORT_SD_STATUS"), ENABLED(AUTO_REPORT_SD_STATUS));
 
+    // LONG_FILENAME_HOST_SUPPORT (M33)
+    cap_line(PSTR("LONG_FILENAME"), ENABLED(LONG_FILENAME_HOST_SUPPORT));
+
     // THERMAL_PROTECTION
     cap_line(PSTR("THERMAL_PROTECTION"), ENABLED(THERMALLY_SAFE));