tinyproxy: add meta.mainProgram

Fix the following warning when using services.tinyproxy:

trace: warning: getExe: Package "tinyproxy-1.11.1" does not have the
meta.mainProgram attribute. We'll assume that the main program has the same
name for now, but this behavior is deprecated, because it leads to surprising
errors when the assumption does not hold. If the package has a main program,
please set `meta.mainProgram` in its definition to make this warning go away.
Otherwise, if the package does not have a main program, or if you don't control
its definition, use getExe' to specify the name to the program, such as
lib.getExe' foo "bar".

pkgs/tools/networking/tinyproxy/default.nix

@@ -39,5 +39,6 @@ stdenv.mkDerivation rec {
     license = licenses.gpl2Only;
     platforms = platforms.all;
     maintainers = [ maintainers.carlosdagos ];
+    mainProgram = "tinyproxy";
   };
 }