highlight: Fix missing .rgb_sp_color in initializers (#9287)

terminal_get_line_attributes() had this bug for a long time, though it
likely had no effect visible to users.

ref #9028
ref 60f845ca55

test/functional/ui/highlight_spec.lua

@@ -797,9 +797,9 @@ describe('CursorLine highlight', function()
       [8] = {bold = true, foreground = Screen.colors.Blue1},
       [9] = {bold = true, reverse = true},
       [10] = {bold = true},
-      [11] = {special = Screen.colors.Grey0, underline = true,
+      [11] = {underline = true,
               background = Screen.colors.LightMagenta},
-      [12] = {bold = true, underline = true, special = Screen.colors.Grey0,
+      [12] = {bold = true, underline = true,
               background = Screen.colors.Red},
     })
   end)