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/truncate.cpp

@@ -26,6 +26,7 @@
 
 #include <LibCore/ArgsParser.h>
 
+#include <fcntl.h>
 #include <stdio.h>
 #include <sys/stat.h>
 #include <unistd.h>