nRF5 SDK16: Ble DFU service not conditionally compiled

While most of nRF5 DSK modules have conditional compiling, such as 

#if NRF_MODULE_ENABLED(BLE_DIS)

the components/ble/ble_services/ble_dfu/*.c lack these macros. In my project which has unbonded dfu service on nRF52832 but no bootloader at all on nRF52811 this wastes about 80 bytes of RAM to static variables and probably some flash to code. It's a relatively minor thing but I came across it when squeezing my app into the available RAM. 

I'll add #if NRF_MODULE_ENABLED(BLE_DFU) on my files, just noting this here for the benefit of others.