LibC: Pull fd_set.h into unistd.h

Author: https://github.com/alimpfard
Commit: 2db4ab80a2
Pull-request: https://github.com/SerenityOS/serenity/pull/5325
Issue: https://github.com/SerenityOS/serenity/issues/90
Reviewed-by: https://github.com/BenWiederhake
Reviewed-by: https://github.com/linusg
Reviewed-by: https://github.com/sunverwerth

Userland/Libraries/LibC/unistd.h

@@ -34,6 +34,7 @@
 #pragma once
 
 #include <errno.h>
+#include <fd_set.h>
 #include <limits.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>