vm_phys: #include vm_extern

Arm64 and powerpc don't include vm_extern.h indirectly in vm_phys.c, which
means that for the sake of those architectures, it must be included explicitly.

Also, fix a set-unused warning that jenkins also found.

Reported by: Jenkins
Fixes: c606ab59e7f9 vm_extern: use standard address checkers everywhere

(cherry picked from commit 01e115ab83a4b82216e02198dbcc7c794a752711)