Add <string.h> to files that needed it.

diff --git a/dlls/dinput/dinput_main.c b/dlls/dinput/dinput_main.c
index 1bfb489..42e7c7a 100644
--- a/dlls/dinput/dinput_main.c
+++ b/dlls/dinput/dinput_main.c
@@ -20,6 +20,7 @@
 
 #include "config.h"
 #include <assert.h>
+#include <string.h>
 
 #include "debugtools.h"
 #include "winbase.h"