Replace use of deprecated C Standard library headers in C++ code.

Change-Id: I9fa6630c7e4bdb2897df34d417635d8c7d8253bc
Signed-off-by: Tyler Chatow <tchatow@gmail.com>
diff --git a/frc971/input/joystick_input.cc b/frc971/input/joystick_input.cc
index c92d567..d0d9bc0 100644
--- a/frc971/input/joystick_input.cc
+++ b/frc971/input/joystick_input.cc
@@ -1,8 +1,7 @@
 #include "frc971/input/joystick_input.h"
 
-#include <string.h>
-
 #include <atomic>
+#include <cstring>
 
 #include "aos/logging/logging.h"
 #include "frc971/input/robot_state_generated.h"