extractor: Fix compile warnings
Following compile warnings showed up when link, mkfifo and mklink weren't defined:
../gnome-autoar/autoar-extractor.c: In function ‘autoar_extractor_do_write_entry’:
../gnome-autoar/autoar-extractor.c:993:17: warning: unused variable ‘uname’ [-Wunused-variable]
993 | const char *uname;
| ^~~~~
../gnome-autoar/autoar-extractor.c:1022:17: warning: unused variable ‘gname’ [-Wunused-variable]
1022 | const char *gname;
| ^~~~~
../gnome-autoar/autoar-extractor.c:1237:1: warning: label ‘applyinfo’ defined but not used [-Wunused-label]
1237 | applyinfo:
| ^~~~~~~~~
../gnome-autoar/autoar-extractor.c:941:7: warning: variable ‘r’ set but not used [-Wunused-but-set-variable]
941 | int r;
| ^