Fix linking when configured with -DENABLE_WALLET=OFF (Qt 6)

[hebasto]

This PR is a part of the transition to Qt 6.

When building with Qt 6 in my dev branch, I encountered a linker error when configured with -DENABLE_WALLET=OFF:

$ cmake -B build -DENABLE_WALLET=OFF -DBUILD_GUI=ON
$ cmake --build build -t bitcoin-qt
<snip>
[100%] Linking CXX executable bitcoin-qt
/usr/bin/ld: libbitcoinqt.a(rpcconsole.cpp.o): in function `QtPrivate::MetaObjectForType<WalletModel const*, void>::metaObjectFunction(QtPrivate::QMetaTypeInterface const*)':
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:903:(.text._ZN9QtPrivate17MetaObjectForTypeIPK11WalletModelvE18metaObjectFunctionEPKNS_18QMetaTypeInterfaceE[_ZN9QtPrivate17MetaObjectForTypeIPK11WalletModelvE18metaObjectFunctionEPKNS_18QMetaTypeInterfaceE]+0x2b): undefined reference to `WalletModel::staticMetaObject'
/usr/bin/ld: libbitcoinqt.a(rpcconsole.cpp.o): in function `QMetaTypeIdQObject<WalletModel const*, 8>::qt_metatype_id()':
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:1313:(.text._ZZN9QtPrivate16QMetaTypeForTypeIPK11WalletModelE17getLegacyRegisterEvENUlvE_4_FUNEv[_ZZN9QtPrivate16QMetaTypeForTypeIPK11WalletModelE17getLegacyRegisterEvENUlvE_4_FUNEv]+0x53): undefined reference to `WalletModel::staticMetaObject'
collect2: error: ld returned 1 exit status
gmake[3]: *** [src/qt/CMakeFiles/bitcoin-qt.dir/build.make:154: src/qt/bitcoin-qt] Error 1
gmake[2]: *** [CMakeFiles/Makefile2:2107: src/qt/CMakeFiles/bitcoin-qt.dir/all] Error 2
gmake[1]: *** [CMakeFiles/Makefile2:2114: src/qt/CMakeFiles/bitcoin-qt.dir/rule] Error 2
gmake: *** [Makefile:998: bitcoin-qt] Error 2

This PR resolves the issue.

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ACK	pablomartin4btc

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[fanquake]

What is the error? Why does it only happen with Qt6?

[hebasto]

What is the error?

I put it to the PR description.

Why does it only happen with Qt6?

Differences in Qt implementation between Qt 5 and Qt 6? (did not dig into the exact code change though)

[furszy]

How the RPCExecutor::request method compiles when this line doesn't exist?

[pablomartin4btc]

I think because the compiler only needs to know WalletModel, it doesn't need the full definition unless you dereference the pointer (eg as in RPCConsole::RPCParseCommandLine -> wallet_model->getWalletName() - which is guarded).

[pablomartin4btc]

tACK 5be34ba

It builds correctly on Qt5 and qt runs fine, with both -DENABLE_WALLET=OFF & -DENABLE_WALLET=ON. When wallet support is off also tested the rpc console and several rpc commands.