patch 8.0.0673: build failure without conceal feature

Problem:    Build failure without conceal feature.
Solution:   Add #ifdef.
diff --git a/src/syntax.c b/src/syntax.c
index da2c876..09013b0 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -1061,7 +1061,9 @@
 
     next_match_idx = -1;
     ++current_line_id;
+#ifdef FEAT_CONCEAL
     next_seqnr = 1;
+#endif
 }
 
 /*