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/tag/Pool.cxx

@@ -24,11 +24,11 @@
 #include "util/StringView.hxx"
 
 #include <cassert>
+#include <cstdint>
 #include <limits>
 
 #include <string.h>
 #include <stdlib.h>
-#include <stdint.h>
 
 Mutex tag_pool_lock;