LibC: Move S_* defines into <fcntl.h>

According to the Single UNIX Specification, Version 2 that's where
those macros should be defined. This fixes the libiconv port.

This also fixes some (but not all) build errors for the diffutils and nano ports.

Userland/Utilities/tee.cpp

@@ -30,6 +30,7 @@
 #include <fcntl.h>
 #include <signal.h>
 #include <stdio.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 static Vector<int> collect_fds(Vector<const char*> paths, bool append, bool* err)