Revert the change to Symbol.map, made in r202274.

Even though we use __sym_compat(), we should list the symbol in
Symbol.map.

ttyslot() is now listed as follows, which seems to do the right thing:

| Symbol table '.dynsym' contains 2755 entries:
|    Num:    Value          Size Type    Bind   Vis      Ndx Name
|    613: 00000000000477b0   121 FUNC    GLOBAL DEFAULT   10 ttyslot@FBSD_1.0

Reported by:	kib

lib/libc/gen/Symbol.map

@@ -272,6 +272,7 @@ FBSD_1.0 {
 	openlog;
 	closelog;
 	setlogmask;
+	ttyslot;
 	ttyname_r;
 	ttyname;
 	timezone;