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/gui.h b/src/gui.h
index 4769716..37cc239 100644
--- a/src/gui.h
+++ b/src/gui.h
@@ -142,6 +142,7 @@
 # define DRAW_ITALIC		0x10	/* draw italic text */
 #endif
 #define DRAW_CURSOR		0x20	/* drawing block cursor (win32) */
+#define DRAW_STRIKE		0x40	/* strikethrough */
 
 /* For our own tearoff menu item */
 #define TEAR_STRING		"-->Detach"