patch 8.0.1038: strike-through text not supported

Problem:    Strike-through text not supported.
Solution:   Add support for the "strikethrough" attribute. (Christian
            Brabandt, Ken Takata)
diff --git a/src/vim.h b/src/vim.h
index 7fea70b..77081e2 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -681,7 +681,8 @@
 #define HL_UNDERCURL		0x10
 #define HL_STANDOUT		0x20
 #define HL_NOCOMBINE		0x40
-#define HL_ALL			0x7f
+#define HL_STRIKETHROUGH	0x80
+#define HL_ALL			0xff
 
 /* special attribute addition: Put message in history */
 #define MSG_HIST		0x1000