replace stdint.h with cstdint

The former is deprecated by C++14. The standard says they are the same:

The header defines all types and macros the same as the C standard library
header<stdint.h>.

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

src/filter/plugins/RouteFilterPlugin.cxx

@@ -53,10 +53,10 @@
 #include "util/WritableBuffer.hxx"
 
 #include <array>
+#include <cstdint>
 #include <stdexcept>
 
 #include <string.h>
-#include <stdint.h>
 #include <stdlib.h>
 
 class RouteFilter final : public Filter {