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/db/plugins/simple/Directory.cxx

@@ -33,7 +33,8 @@
 #include "util/Alloc.hxx"
 #include "util/DeleteDisposer.hxx"
 
-#include <assert.h>
+#include <cassert>
+
 #include <string.h>
 #include <stdlib.h>
 

src/db/plugins/upnp/UpnpDatabasePlugin.cxx

@@ -40,10 +40,10 @@
 #include "util/RecursiveMap.hxx"
 #include "util/SplitString.hxx"
 
+#include <cassert>
 #include <string>
 #include <utility>
 
-#include <assert.h>
 #include <string.h>
 
 static const char *const rootid = "0";