crypto: move null_pkey/null_skey to the cpp file

src/crypto/crypto.cpp

@@ -70,6 +70,9 @@ namespace crypto {
 #include "random.h"
   }
 
+  const crypto::public_key null_pkey = boost::value_initialized<crypto::public_key>();
+  const crypto::secret_key null_skey = boost::value_initialized<crypto::secret_key>();
+
   static inline unsigned char *operator &(ec_point &point) {
     return &reinterpret_cast<unsigned char &>(point);
   }

src/crypto/crypto.h

@@ -277,8 +277,8 @@ namespace crypto {
     epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
   }
 
-  const static crypto::public_key null_pkey = boost::value_initialized<crypto::public_key>();
-  const static crypto::secret_key null_skey = boost::value_initialized<crypto::secret_key>();
+  const extern crypto::public_key null_pkey;
+  const extern crypto::secret_key null_skey;
 }
 
 CRYPTO_MAKE_HASHABLE(public_key)