[Backport staging-26.05] python3Packages.mypy: fix cross compilation (#530139)

pkgs/development/python-modules/mypy/default.nix

@@ -13,8 +13,10 @@
   types-psutil,
   types-setuptools,
 
-  # propagates
+  # nativeBuildInputs + propagates
   librt,
+
+  # propagates
   mypy-extensions,
   tomli,
   typing-extensions,
@@ -50,6 +52,10 @@ buildPythonPackage rec {
     rev-prefix = "v";
   };
 
+  nativeBuildInputs = [
+    librt
+  ];
+
   build-system = [
     mypy-extensions
     pathspec