INSTALL: update obsolete doc, as suggested by Doug Clapp (doug.clapp AT triad.rr.com)

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

coreutils/sync.c

@@ -14,7 +14,7 @@
 /* This is a NOFORK applet. Be very careful! */
 
 int sync_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
-int sync_main(int argc UNUSED_PARAM, char **argv)
+int sync_main(int argc UNUSED_PARAM, char **argv IF_NOT_DESKTOP(UNUSED_PARAM))
 {
 	/* coreutils-6.9 compat */
 	bb_warn_ignoring_args(argv[1]);