Fix GCC 7.4.0 warning

Warning: enumeral and non-enumeral type in conditional expression.

src/wallet/wallet.cpp

@@ -2266,7 +2266,7 @@ void MaybeResendWalletTxs()
 CWallet::Balance CWallet::GetBalance(const int min_depth, bool avoid_reuse) const
 {
     Balance ret;
-    isminefilter reuse_filter = avoid_reuse ? 0 : ISMINE_USED;
+    isminefilter reuse_filter = avoid_reuse ? ISMINE_NO : ISMINE_USED;
     {
         auto locked_chain = chain().lock();
         LOCK(cs_wallet);