conversations.c: add a cast needed to silence an error in clang

buf_putc wants a char (no specified sign), and we're passing an int with
the high bit very intentionally set.  We need to cast the resulting
expression to (char).

imap/conversations.c

@@ -2267,15 +2267,15 @@ static int conversations_guid_setitem(struct conversations_state *state,
     if (add) {
         struct buf val = BUF_INITIALIZER;
         if (state->folders_byname) {
-            buf_putc(&val, 0x80 | CONV_GUIDREC_BYNAME_VERSION);
+            buf_putc(&val, (char) (0x80 | CONV_GUIDREC_BYNAME_VERSION));
             buf_appendbit64(&val, cid);
             buf_appendbit32(&val, system_flags);
             buf_appendbit32(&val, internal_flags);
             buf_appendbit64(&val, (bit64)internaldate);
         }
         /* When bumping the G value version, make sure to update _guid_cb */
         else {
-            buf_putc(&val, 0x80 | CONV_GUIDREC_VERSION);
+            buf_putc(&val, (char) (0x80 | CONV_GUIDREC_VERSION));
             buf_appendbit64(&val, cid);
             buf_appendbit32(&val, system_flags);
             buf_appendbit32(&val, internal_flags);