btanks: fix build

commit 7aeac03 (PR #386495) dropped libX11 and libGLU from propagatedBuildInputs in SDL_compat, and #389106 pointed SDL to SDL_compat, which broke this build

Hydra fail: https://hydra.nixos.org/build/292729441

Grimmauld 57988f96 2341cd6a

+4
+4
pkgs/by-name/bt/btanks/package.nix
··· 6 fetchpatch, 7 fetchurl, 8 libGL, 9 libvorbis, 10 lua, 11 pkg-config, 12 scons, ··· 62 SDL_image 63 expat 64 libGL 65 libvorbis 66 lua 67 smpeg
··· 6 fetchpatch, 7 fetchurl, 8 libGL, 9 + libGLU, 10 libvorbis, 11 + libX11, 12 lua, 13 pkg-config, 14 scons, ··· 64 SDL_image 65 expat 66 libGL 67 + libGLU 68 + libX11 69 libvorbis 70 lua 71 smpeg