stub: Fix build failure with --enable-user --disable-system --enable-tools

stub: Fix build failure with --enable-user --disable-system --enable-tools

Configuring "--enable-user --disable-system --enable-tools" causes the
build failure with the following information:

/usr/bin/ld: libhwcore.a.p/hw_core_qdev.c.o: in function `device_finalize':
/qemu/build/../hw/core/qdev.c:688: undefined reference to `qapi_event_send_device_deleted'
collect2: error: ld returned 1 exit status

To fix the above issue, add qdev.c stub when build with `have_tools`.

With this fix, QEMU could be successfully built in the following cases:
 --enable-user --disable-system --enable-tools
 --enable-user --disable-system --disable-tools
 --enable-user --disable-system

Cc: qemu-stable@nongnu.org
Fixes: 388b849fb6c3 ("stubs: avoid duplicate symbols in libqemuutil.a")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2766
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250121154318.214680-1-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>