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

diff --git a/dlls/kernel/kernel_main.c b/dlls/kernel/kernel_main.c
index afdc2b4..36f6f8c 100644
--- a/dlls/kernel/kernel_main.c
+++ b/dlls/kernel/kernel_main.c
@@ -4,6 +4,7 @@
 
 #include <assert.h>
 #include <ctype.h>
+#include <string.h>
 
 #include "winbase.h"
 #include "wine/winbase16.h"