monitor: Include missing 'qemu/lockable.h' header

Files in monitor/ use the QEMU_LOCK_GUARD() macros, which
are declared in "qemu/lockable.h". Include the latter to
avoid when refactoring unrelated headers:

  ../monitor/fds.c:146:5: error: call to undeclared function 'QEMU_LOCK_GUARD'
    146 |     QEMU_LOCK_GUARD(&mon->mon_lock);
        |     ^
  ../monitor/monitor.c:176:5: error: call to undeclared function 'QEMU_LOCK_GUARD'
    176 |     QEMU_LOCK_GUARD(&mon->mon_lock);
        |     ^
  ../monitor/qmp.c:164:5: error: call to undeclared function 'WITH_QEMU_LOCK_GUARD'
    164 |     WITH_QEMU_LOCK_GUARD(&mon->mon_lock) {
        |     ^

Fixes: 0210c3b39be ("monitor: Use LOCK_GUARD macros")
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20260812211708.92824-9-philmd@oss.qualcomm.com>