A minor bug exists in SDK 15.0.0, components/ble/nrf_ble_qwr/nrf_ble_qwr.h, which arises when compiled using a C++ compiler.
The following C++ conditional at the top of the file should be inside the NRF_BLE_QUEUE_WRITES_H__ header guard.
#ifdef __cplusplus
extern "C" {
#endif
A patch to address this issue is attached.