Make all header files self-suffienct

Almost all the headers in cryptsetup are self-suffienct (in that they
compile on their own). By including <stddef.h>, <stdint.h>, or
<sys/types.h>, all headers will now compile on their own.

This is useful for importing cryptsetup into Bazel/Blaze.

lib/utils_keyring.h

@@ -22,6 +22,8 @@
 #ifndef _UTILS_KEYRING
 #define _UTILS_KEYRING
 
+#include <stddef.h>
+
 int keyring_check(void);
 
 int keyring_get_passphrase(const char *key_desc,