build: test for timingsafe_bcmp

Code introduced in #15649 added usage of `timingsafe_bcmp()`, if
available, otherwise falling back to our own implementation. However
the relevant build system check was never added, so currently, we'll
always just use our implementation, as HAVE_TIMINGSAFE_BCMP will never
be defined.

Add the check for timingsafe_bcmp. Note that as far as I'm aware, it's
only available on OpenBSD.

src/crypto/chacha_poly_aead.cpp

@@ -2,6 +2,10 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#if defined(HAVE_CONFIG_H)
+#include <config/bitcoin-config.h>
+#endif
+
 #include <crypto/chacha_poly_aead.h>
 
 #include <crypto/poly1305.h>