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/system/EpollFD.hxx

@@ -32,8 +32,9 @@
 
 #include "UniqueFileDescriptor.hxx"
 
+#include <cstdint>
+
 #include <sys/epoll.h>
-#include <stdint.h>
 
 /**
  * A class that wraps Linux epoll.

src/system/FileDescriptor.cxx

@@ -31,8 +31,8 @@
 #include "system/Error.hxx"
 
 #include <cassert>
+#include <cstdint>
 
-#include <stdint.h>
 #include <sys/stat.h>
 #include <fcntl.h>