Remove unit64_t format specifier warnings

There are format build warnings present with GCC 4.8.1 on Linux.

```
src/core/global.c: In function ‘nn_global_submit_counter’:
src/core/global.c:904:13: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 5 has type ‘uint64_t’ [-Wformat=]
             s->socket_name, name, value);
             ^
src/core/global.c:919:17: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 8 has type ‘uint64_t’ [-Wformat=]
                 timebuf, value);
                 ^
src/core/global.c:923:17: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 8 has type ‘uint64_t’ [-Wformat=]
                 timebuf, value);
                 ^
```

This patch explicitly casts 'value' to match the format specifier, and results in a clean build.