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
 }
 
 /*
diff --git a/src/version.c b/src/version.c
index d15b7db..b9f309d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    673,
+/**/
     672,
 /**/
     671,