Fixes compile with libc++ on linux:

```
In file included from ../tests/cc/test_array_table.cc:17:
In file included from ../src/cc/api/BPF.h:24:
../src/cc/api/BPFTable.h:118:71: error: use of undeclared identifier 'errno'
      return StatusTuple(-1, "Error getting value: %s", std::strerror(errno));
```