Use local SHA512 header

Actually use the local private SHA512 header instead of relying on the
OpenSSL one for no good reason. Add definition for expected macro
SHA512_DIGEST_LENGTH.

src/getentropy_linux.c

@@ -47,7 +47,8 @@
 #include <errno.h>
 #include <unistd.h>
 #include <time.h>
-#include <openssl/sha.h>
+
+#include "hash/sha512.h"
 
 #include <linux/types.h>
 #include <linux/random.h>