ec: Fix build on CentOS.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

src/curve25519.c

@@ -37,6 +37,7 @@
 #include "libssh/crypto.h"
 #include "libssh/dh.h"
 #include "libssh/pki.h"
+#include "libssh/bignum.h"
 
 /** @internal
  * @brief Starts curve25519-sha256@libssh.org key exchange

src/ecdh.c

@@ -26,6 +26,7 @@
 #include "libssh/buffer.h"
 #include "libssh/ssh2.h"
 #include "libssh/pki.h"
+#include "libssh/bignum.h"
 
 #ifdef HAVE_ECDH
 #include <openssl/ecdh.h>