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/terminal/highlight_spec.lua

@@ -120,7 +120,7 @@ describe('terminal window highlighting with custom palette', function()
     clear()
     screen = Screen.new(50, 7)
     screen:set_default_attr_ids({
-      [1] = {foreground = 1193046, special = Screen.colors.Black},
+      [1] = {foreground = tonumber('0x123456')},
       [2] = {foreground = 12},
       [3] = {bold = true, reverse = true},
       [5] = {background = 11},