[crypto] compilation fix for clang10 -Wdeprecated-copy (#7487)

Deleting a copy constructor without specifying an explicit assignment
operator causes an error in clang10 (-Wdeprecated-copy). Since
explicit assignment operator defined in
OPENTHREAD_CONFIG_PLATFORM_KEY_REFERENCES_ENABLE case, added the copy
constructor deletion also under the same. If the macro is not enabled,
implicit copy constructor and assignment operator would take effect.

src/core/mac/mac_types.hpp

@@ -469,6 +469,8 @@ public:
      *
      */
     KeyMaterial &operator=(const KeyMaterial &aOther);
+
+    KeyMaterial(const KeyMaterial &) = delete;
 #endif
 
     /**
@@ -539,8 +541,6 @@ public:
      */
     bool operator==(const KeyMaterial &aOther) const;
 
-    KeyMaterial(const KeyMaterial &) = delete;
-
 private:
 #if OPENTHREAD_CONFIG_PLATFORM_KEY_REFERENCES_ENABLE
     static constexpr KeyRef kInvalidKeyRef = Crypto::Storage::kInvalidKeyRef;