common/ipc: skip variables for native code on nacl

[illwieckz]

Something I found while unifying compiler flags between native builds and nacl builds.

Unvanquished/daemon/src/common/IPC/Primitives.cpp:85:18: warning: 
      unused variable 'NACL_DESC_TYPE_END_TAG' [-Wunused-const-variable]
static const int NACL_DESC_TYPE_END_TAG  = (0xff);
                 ^
Unvanquished/daemon/src/common/IPC/Primitives.cpp:98:23: warning: 
      unused variable 'NACL_HANDLE_TRANSFER_PROTOCOL' [-Wunused-const-variable]
static const uint32_t NACL_HANDLE_TRANSFER_PROTOCOL = 0xd3c0de01;
                      ^
2 warnings generated.