sim: fix build breakage in process.cc after brandon@11801

sim: fix build breakage in process.cc after brandon@11801

Seeing build breakage after brandon@11801:

 [     CXX] X86/sim/process.cc -> .o build/X86/sim/process.cc:137:64:
error: field '_pid' is uninitialized when used here
[-Werror,-Wuninitialized] static_cast<PageTableBase *>(new
ArchPageTable(name(), _pid, system)) : ^ build/X86/sim/process.cc:138:64:
error: field '_pid' is uninitialized when used here
[-Werror,-Wuninitialized] static_cast<PageTableBase *>(new
FuncPageTable(name(), _pid))), ^ 2 errors generated.

Testing Done: Compiles now on FreeBSD 10 with clang.

Reviewed at http://reviews.gem5.org/r/3804/

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>