amd64: pmap.h put a guard around a pcpu.h function

pmap_get_pcid() calls zpcpu_get() which is defined in pcpu.h.
It is unclear why we do not include that header but like right
above the change add another guard around pmap_get_pcid().
This allows some LinuxKPI headers to compile again.

Suggested by: markj
MFC after: 10 days