patch 9.1.1391: Vim does not have a vertical tabpanel

Problem:  Vim does not have a tabpanel
Solution: include the tabpanel feature
          (Naruhiko Nishino, thinca)

closes: #17263

Co-authored-by: thinca <thinca@gmail.com>
Signed-off-by: Naruhiko Nishino <naru123456789@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/option.h b/src/option.h
index bb1226e..e78a7cb 100644
--- a/src/option.h
+++ b/src/option.h
@@ -984,7 +984,15 @@
 #define SWB_NEWTAB		0x008
 #define SWB_VSPLIT		0x010
 #define SWB_USELAST		0x020
+
 EXTERN char_u	*p_spk;		// 'splitkeep'
+
+#if defined(FEAT_TABPANEL)
+EXTERN char_u	*p_tpl;		// 'tabpanel'
+EXTERN long	p_stpl;		// 'showtabpanel'
+EXTERN char_u	*p_tplo;	// 'tabpanelopt'
+#endif
+
 #ifdef FEAT_SYN_HL
 EXTERN char_u	*p_syn;		// 'syntax'
 #endif