gimp: fixup build, missing xz libs

https://hydra.nixos.org/build/305228462/nixlog/1/tail

+2
+2
pkgs/applications/graphics/gimp/2.0/default.nix
··· 28 libwmf, 29 zlib, 30 libzip, 31 ghostscript, 32 aalib, 33 shared-mime-info, ··· 133 libwmf 134 zlib 135 libzip 136 ghostscript 137 aalib 138 shared-mime-info
··· 28 libwmf, 29 zlib, 30 libzip, 31 + xz, 32 ghostscript, 33 aalib, 34 shared-mime-info, ··· 134 libwmf 135 zlib 136 libzip 137 + xz 138 ghostscript 139 aalib 140 shared-mime-info