ble_mesh_light/ws2812: Only compile if USE_NEOPIXEL is set.
ble_mesh_light/ws2812: Only compile if USE_NEOPIXEL is set.

Without this builds for platforms other than NRFx fail even when
USE_NEOPIXEL is set to 0 due to inclusion of NRFx header files.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble