GCS_FTP fails @ line 570 for ESP32buzz

[Nic-Gould]

Bug report

Issue details
AP_Filesystem defines DT_LNK ONLY for Chibi OS, but GCS_FTP looks for it on line 570 regardless of boardtype. Copying the #defines into AP_Filesystem_ESP32 or GCS_FTP fixes the problem.

Version
Master

Platform
[ ] Copter

Airframe type
Quad

Hardware type
Building on linux desktop, target esp32buzz

Logs
../../libraries/GCS_MAVLink/GCS_FTP.cpp: In static member function 'static int GCS_MAVLINK::gen_dir_entry(char*, size_t, const char*, const dirent*)':
../../libraries/GCS_MAVLink/GCS_FTP.cpp:570:70: error: 'DT_LNK' was not declared in this scope
const bool is_file = entry->d_type == DT_REG || entry->d_type == DT_LNK;