clang-tidy: add explicit deleted constructors

Found with cppcoreguidelines-special-member-functions

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/filter/Observer.cxx

@@ -43,6 +43,9 @@ public:
 		observer.proxy = nullptr;
 	}
 
+	PreparedProxy(const PreparedProxy &) = delete;
+	PreparedProxy &operator=(const PreparedProxy &) = delete;
+
 	void Clear([[maybe_unused]] Proxy *_child) noexcept {
 		assert(child == _child);
 		child = nullptr;
@@ -67,6 +70,9 @@ public:
 		parent.Clear(this);
 	}
 
+	Proxy(const Proxy &) = delete;
+	Proxy &operator=(const Proxy &) = delete;
+
 	Filter *Get() noexcept {
 		return filter.get();
 	}