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/client/Idle.cxx

@@ -22,7 +22,7 @@
 #include "Response.hxx"
 #include "Idle.hxx"
 
-#include <assert.h>
+#include <cassert>
 
 static void
 WriteIdleResponse(Response &r, unsigned flags) noexcept

src/client/List.cxx

@@ -20,7 +20,7 @@
 #include "List.hxx"
 #include "util/DeleteDisposer.hxx"
 
-#include <assert.h>
+#include <cassert>
 
 ClientList::~ClientList() noexcept
 {

src/client/New.cxx

@@ -29,7 +29,7 @@
 #include "net/ToString.hxx"
 #include "Log.hxx"
 
-#include <assert.h>
+#include <cassert>
 
 static constexpr char GREETING[] = "OK MPD " PROTOCOL_VERSION "\n";
 

src/client/Subscribe.cxx

@@ -21,7 +21,7 @@
 #include "Partition.hxx"
 #include "IdleFlags.hxx"
 
-#include <assert.h>
+#include <cassert>
 
 Client::SubscribeResult
 Client::Subscribe(const char *channel) noexcept