mem: Removed "using namespace std;" from src/mem/packet.cc

mem: Removed "using namespace std;" from src/mem/packet.cc

To avoid unintentional variable capture, all std calls must be
prefixed. These are the identifiers which are in the std
namespace (according to
https://en.cppreference.com/w/cpp/symbol_index), but that will remain
unprefixed with this change:

int8_t int16_t int32_t int64_t
uint8_t uint16_t uint32_t uint64_t

The (u)int types are included from the packet header file, which
includes <inttypes.h>, where they occur in the global namespace. They
are in the std namespace in <cinttypes>/<cstdint>.

There is an occurrence of "set" in this file, which is "Packet::set"
and not "std::set", so it is not prefixed with the std namespace

Change-Id: I7f6c0b61b09658e224fe31a9f73150b81861d6f8
Reviewed-on: https://gem5-review.googlesource.com/11809
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>