shutdown: ifdef for NuttX builds without work queue support
shutdown: ifdef for NuttX builds without work queue support

For example on px4cannode-v1_default.

mje-nz/PX4-Firmware,jlecoeur/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,PX4/Firmware,Aerotenna/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,acfloria/Firmware,PX4/Firmware,Aerotenna/Firmware,PX4/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,acfloria/Firmware,jlecoeur/Firmware,dagar/Firmware,krbeverx/Firmware,acfloria/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,Aerotenna/Firmware,acfloria/Firmware,krbeverx/Firmware,dagar/Firmware,dagar/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware