replace assert.h with cassert

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.

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

src/thread/Thread.hxx

@@ -23,14 +23,14 @@
 #include "util/BindMethod.hxx"
 #include "util/Compiler.h"
 
+#include <cassert>
+
 #ifdef _WIN32
 #include <windows.h>
 #else
 #include <pthread.h>
 #endif
 
-#include <assert.h>
-
 class Thread {
 	typedef BoundMethod<void() noexcept> Function;
 	const Function f;