Add explicit rule for class.c to vms/descrip_mms.template

Without the explicit rule, it still gets compiled, but without
PERL_CORE defined, which makes class_cleanup_definition get
interpreted as a function rather than a macro, which causes the
link to fail with an undefined symbol error.

Author: craigberry

vms/descrip_mms.template

@@ -612,6 +612,8 @@ builtin$(O) : builtin.c $(h)
 	$(CC) $(CORECFLAGS) $(MMS$SOURCE)
 caretx$(O) : caretx.c $(h)
 	$(CC) $(CORECFLAGS) $(MMS$SOURCE)
+class$(O) : class.c $(h)
+	$(CC) $(CORECFLAGS) $(MMS$SOURCE)
 deb$(O) : deb.c $(h)
 	$(CC) $(CORECFLAGS) $(MMS$SOURCE)
 doio$(O) : doio.c $(h)