linux_4_15: fix a partial version downgrade on staging merge

pkgs/os-specific/linux/kernel/linux-4.15.nix

···
 with stdenv.lib;
 
 buildLinux (args // rec {
-  version = "4.15.1";
+  version = "4.15.2";
 
   # modDirVersion needs to be x.y.z, will automatically add .0 if needed
   modDirVersion = concatStrings (intersperse "." (take 3 (splitString "." "${version}.0")));