Replace SHA256 implementation with one from LibreSSL

They have cleaner code than OpenSSL :)

from here: https://github.com/libressl/openbsd/tree/master/src/lib/libcrypto
and https://github.com/libressl/portable

src/util/srp.cpp

@@ -50,7 +50,7 @@
 	#include <mini-gmp.h>
 #endif
 
-#include <util/sha2.h>
+#include "util/sha256.h"
 
 #include "srp.h"
 //#define CSRP_USE_SHA1