liquidwar: use 1.6 branch of libpng

Seems to work fine. /cc maintainer @7c6f434c.

pkgs/top-level/all-packages.nix

@@ -7487,14 +7487,13 @@ let
 
   liquidwar = builderDefsPackage ../games/liquidwar {
     inherit (xlibs) xproto libX11 libXrender;
-    inherit gmp mesa libjpeg
+    inherit gmp mesa libjpeg libpng
       expat gettext perl
       SDL SDL_image SDL_mixer SDL_ttf
       curl sqlite
       libogg libvorbis libcaca csound cunit
       ;
     guile = guile_1_8;
-    libpng = libpng15; # 0.0.13 needs libpng 1.2--1.5
   };
 
   log4cpp = callPackage ../development/libraries/log4cpp { };