common: fix compilation warnings in numa.cc

common: fix compilation warnings in numa.cc

```
[141/1591] Building CXX object src/common/CMakeFiles/common-common-objs.dir/numa.cc.o
In file included from ../src/include/types.h:63,
                 from ../src/include/stringify.h:7,
                 from ../src/common/numa.cc:10:
../src/common/numa.cc: In function ‘std::string get_task_comm(pid_t)’:
../src/common/numa.cc:176:12: warning: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘long int’} and ‘long unsigned int’ [-Wsign-compare]
  176 |   assert(n <= sizeof(name));
      |          ~~^~~~~~~~~~~~~~~
../src/common/numa.cc: At global scope:
../src/common/numa.cc:159:20: warning: ‘std::string get_task_comm(pid_t)’ defined but not used [-Wunused-function]
  159 | static std::string get_task_comm(pid_t tid)
      |                    ^~~~~~~~~~~~~

```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>