sys/pcpu.h: Revert change introduced in r303890

device_t is not defined outside the kernel but this header is used by
eg. libkvm or vmstat(8). Thus, r303890 broke the build.

So let's restore struct device here until a longer term solution is
found.

Reported by: Michael Butler <imb@protected-networks.net>, Jenkins
MFC after: 3 days
MFC with: r303890