drivers/nrf51: add missing include of os_cputime.h
#include <assert.h>
#include "syscfg/syscfg.h"
#include "os/os.h"
+/* Keep os_cputime explicitly to enable build on non-Mynewt platforms */
+#include "os/os_cputime.h"
#include "ble/xcvr.h"
#include "nimble/ble.h"
#include "nimble/nimble_opt.h"
