patch 8.0.0673: build failure without conceal feature

Problem:    Build failure without conceal feature.
Solution:   Add #ifdef.

src/syntax.c

@@ -1061,7 +1061,9 @@ syn_start_line(void)
 
     next_match_idx = -1;
     ++current_line_id;
+#ifdef FEAT_CONCEAL
     next_seqnr = 1;
+#endif
 }
 
 /*

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    673,
 /**/
     672,
 /**/