wallet: make CanGenerateKeys() const

This method simply checks if HD is or can be enabled and does not require mutability.

src/wallet/scriptpubkeyman.cpp

@@ -973,7 +973,7 @@ void LegacyScriptPubKeyMan::LoadKeyPool(int64_t nIndex, const CKeyPool &keypool)
         mapKeyMetadata[keyid] = CKeyMetadata(keypool.nTime);
 }
 
-bool LegacyScriptPubKeyMan::CanGenerateKeys()
+bool LegacyScriptPubKeyMan::CanGenerateKeys() const
 {
     // A wallet can generate keys if it has an HD seed (IsHDEnabled) or it is a non-HD wallet (pre FEATURE_HD)
     LOCK(cs_KeyStore);