strace: split man pages off

strace is on every NixOS system. Some people may want to skip `man`
pages to save a bit of closure size, so it makes sense to also do it on
a package like this.

(cherry picked from commit 240d333419)

pkgs/by-name/st/strace/package.nix

@@ -20,6 +20,11 @@ stdenv.mkDerivation rec {
 
   separateDebugInfo = true;
 
+  outputs = [
+    "out"
+    "man"
+  ];
+
   depsBuildBuild = [ buildPackages.stdenv.cc ];
   nativeBuildInputs = [ perl ];