Merge bitcoin-core/gui#837: qt6: Fix linking when configured with `-DENABLE_WALLET=OFF`

5be34bacf6 qt: Fix linking when configured with `-DENABLE_WALLET=OFF` (Hennadii Stepanov)

Pull request description:

  Split from https://github.com/bitcoin/bitcoin/pull/30997.

  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.

ACKs for top commit:
  promag:
    ACK 5be34bacf6, all other changes in 33657e1c958146312e4c68765a92871920401396 are not required, makes the code harder to read.
  pablomartin4btc:
    tACK 5be34bacf6

Tree-SHA512: d10da665384e6539b8e9106dc905ba30e9e57cd4603fc7e5eb893fc899f55f26889c570687fa5daf55c6fc5bf4fcfcfcd9b70822cfe637f31f9151bb653b7941

src/qt/rpcconsole.cpp

@@ -16,7 +16,9 @@
 #include <qt/guiutil.h>
 #include <qt/peertablesortproxy.h>
 #include <qt/platformstyle.h>
+#ifdef ENABLE_WALLET
 #include <qt/walletmodel.h>
+#endif // ENABLE_WALLET
 #include <rpc/client.h>
 #include <rpc/server.h>
 #include <util/strencodings.h>