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/lib/curl/Delegate.cxx

@@ -30,10 +30,9 @@
 #include "Delegate.hxx"
 #include "Parser.hxx"
 
+#include <cassert>
 #include <utility>
 
-#include <assert.h>
-
 void
 DelegateCurlResponseHandler::OnHeaders(unsigned status,
 				       std::multimap<std::string, std::string> &&headers)

src/lib/curl/Global.cxx

@@ -35,7 +35,7 @@
 #include "util/RuntimeError.hxx"
 #include "util/Domain.hxx"
 
-#include <assert.h>
+#include <cassert>
 
 static constexpr Domain curlm_domain("curlm");
 

src/lib/curl/Init.cxx

@@ -32,7 +32,7 @@
 #include "event/Call.hxx"
 #include "thread/Mutex.hxx"
 
-#include <assert.h>
+#include <cassert>
 
 Mutex CurlInit::mutex;
 unsigned CurlInit::ref;

src/lib/curl/Request.cxx

@@ -40,8 +40,8 @@
 #include <curl/curl.h>
 
 #include <algorithm>
+#include <cassert>
 
-#include <assert.h>
 #include <string.h>
 
 CurlRequest::CurlRequest(CurlGlobal &_global,