src: add new and align include guards in header files (#480)

Make sure all include guards exist and follow the same format.

src/crypto.h

@@ -1,3 +1,5 @@
+#ifndef __LIBSSH2_CRYPTO_H
+#define __LIBSSH2_CRYPTO_H
 /* Copyright (C) 2009, 2010 Simon Josefsson
  * Copyright (C) 2006, 2007 The Written Word, Inc.  All rights reserved.
  * Copyright (C) 2010-2019 Daniel Stenberg
@@ -35,8 +37,6 @@
  * USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
  * OF SUCH DAMAGE.
  */
-#ifndef LIBSSH2_CRYPTO_H
-#define LIBSSH2_CRYPTO_H
 
 #ifdef LIBSSH2_OPENSSL
 #include "openssl.h"
@@ -245,4 +245,4 @@ int _libssh2_pub_priv_keyfilememory(LIBSSH2_SESSION *session,
                                     size_t privatekeydata_len,
                                     const char *passphrase);
 
-#endif
+#endif /* __LIBSSH2_CRYPTO_H */