Make CKey::Load references const

No change in behavior, this just prevents CKey::Load arguments from looking
like outputs.

src/key.cpp

@@ -233,7 +233,7 @@ bool CKey::SignCompact(const uint256 &hash, std::vector<unsigned char>& vchSig)
     return true;
 }
 
-bool CKey::Load(CPrivKey &privkey, CPubKey &vchPubKey, bool fSkipCheck=false) {
+bool CKey::Load(const CPrivKey &privkey, const CPubKey &vchPubKey, bool fSkipCheck=false) {
     if (!ec_privkey_import_der(secp256k1_context_sign, (unsigned char*)begin(), privkey.data(), privkey.size()))
         return false;
     fCompressed = vchPubKey.IsCompressed();

src/key.h

@@ -135,7 +135,7 @@ public:
     bool VerifyPubKey(const CPubKey& vchPubKey) const;
 
     //! Load private key and check that public key matches.
-    bool Load(CPrivKey& privkey, CPubKey& vchPubKey, bool fSkipCheck);
+    bool Load(const CPrivKey& privkey, const CPubKey& vchPubKey, bool fSkipCheck);
 };
 
 struct CExtKey {