header.pl: Add reference to colors.txt into header.pl

header.pl: Add reference to colors.txt into header.pl

- With change of common css entries into header.pl with commit
   https://git.ipfire.org/?p=ipfire-2.x.git;a=commit;h=b52a84ddc77f9db7e4ad7b129f21fbf528c6f990
   I had missed that color20 and color22 etc, that come from colors.txt, are not known in
   header.pl so that the alternate colours in tables were missing. Even though I tested the
   previous commit I obviously did not look well enough because I missed that the tables
   had no alternate colours. I just found it now when looking at Core Update 174 Testing.
- Confirmed by copying this version of header.pl into my CU174 Testing system and the
   alternate colour rows came back again as they should be. I am sure now that they are
   correct.

Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Bernhard Bitsch <bbitsch@ipfire.org>
Tested-by: Bernhard Bitsch <bbitsch@ipfire.org>