nixos/systemd-initrd: honor the enable option in contents

The enable attribute of `boot.initrd.systemd.contents.<name>` was
ignored for building initrd storePaths. This resulted in building
derivations for the initrd even if it was disabled.

Found while testing a to build a nixos system with a kernel without
lodable modules[0]

[0]: https://github.com/NixOS/nixpkgs/pull/411792

(cherry picked from commit 99da5ec2bc)

nixos/modules/system/boot/systemd/initrd.nix

@@ -133,7 +133,9 @@ let
     name = "initrd-${kernel-name}";
     inherit (config.boot.initrd) compressor compressorArgs prepend;
 
-    contents = lib.filter ({ source, ... }: !lib.elem source cfg.suppressedStorePaths) cfg.storePaths;
+    contents = lib.filter (
+      { source, enable, ... }: (!lib.elem source cfg.suppressedStorePaths) && enable
+    ) cfg.storePaths;
   };
 
 in