patch 9.0.1798: The 'syntax' option has no completion.

Problem:  The 'syntax' option has no completion.
Solution: Add syntax option completion.

closes: #12900

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Doug Kearns <dougkearns@gmail.com>
diff --git a/src/option.c b/src/option.c
index 15febb7..095653d 100644
--- a/src/option.c
+++ b/src/option.c
@@ -7374,6 +7374,14 @@
 
     xp->xp_pattern = p + 1;
 
+#ifdef FEAT_SYN_HL
+    if (options[opt_idx].var == (char_u *)&p_syn)
+    {
+	xp->xp_context = EXPAND_OWNSYNTAX;
+	return;
+    }
+#endif
+
     if (flags & P_EXPAND)
     {
 	p = options[opt_idx].var;