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/queue/IdTable.hxx

@@ -23,8 +23,7 @@
 #include "util/Compiler.h"
 
 #include <algorithm>
-
-#include <assert.h>
+#include <cassert>
 
 /**
  * A table that maps id numbers to position numbers.

src/queue/Playlist.cxx

@@ -25,7 +25,7 @@
 #include "SingleMode.hxx"
 #include "Log.hxx"
 
-#include <assert.h>
+#include <cassert>
 
 void
 playlist::TagModified(DetachedSong &&song) noexcept

src/queue/Queue.hxx

@@ -25,9 +25,9 @@
 #include "SingleMode.hxx"
 #include "util/LazyRandomEngine.hxx"
 
+#include <cassert>
 #include <utility>
 
-#include <assert.h>
 #include <stdint.h>
 
 class DetachedSong;