packet_crypt: Move secure_memcmp() to a shared source

Move the secure_memcmp() function to a shared source to make it
available internally for other crypto implementations.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

src/CMakeLists.txt

@@ -112,6 +112,7 @@ set(libssh_SRCS
   config.c
   connect.c
   connector.c
+  crypto_common.c
   curve25519.c
   dh.c
   ecdh.c