security+util: mark all pimpl unique_ptrs const

Change-Id: I3966c4988dceb49c204e23967ba90729877bddf1
diff --git a/src/security/tpm/back-end-osx.hpp b/src/security/tpm/back-end-osx.hpp
index 29465c8..8310984 100644
--- a/src/security/tpm/back-end-osx.hpp
+++ b/src/security/tpm/back-end-osx.hpp
@@ -143,7 +143,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 } // namespace tpm