rtla: Fix missing unistd include

rtla: Fix missing unistd include

Compiling RTLA 7.1.x with GCC 16 and uClibc as standard library fails
with these errors:

src/common.c: In function ‘set_signals’:
src/common.c:40:17: error: implicit declaration of function ‘alarm’ [-Wimplicit-function-declaration]
   40 |                 alarm(params->duration);
      |                 ^~~~~
src/common.c: In function ‘common_apply_config’:
src/common.c:187:44: error: implicit declaration of function ‘getpid’; did you mean ‘getpt’? [-Wimplicit-function-declaration]
  187 |                 retval = sched_setaffinity(getpid(), sizeof(params->hk_cpu_set),
      |                                            ^~~~~~
      |                                            getpt
In file included from src/common.c:9:
src/common.c: In function ‘run_tool’:
src/common.c:262:19: error: implicit declaration of function ‘sysconf’; did you mean ‘sscanf’? [-Wimplicit-function-declaration]
  262 |         nr_cpus = get_nprocs_conf();
      |                   ^~~~~~~~~~~~~~~
src/common.c:262:19: error: ‘_SC_NPROCESSORS_CONF’ undeclared (first use in this function)
  262 |         nr_cpus = get_nprocs_conf();
      |                   ^~~~~~~~~~~~~~~
src/common.c:262:19: note: each undeclared identifier is reported only once for each function it appears in
src/common.c:370:17: error: implicit declaration of function ‘sleep’ [-Wimplicit-function-declaration]
  370 |                 sleep(1);
      |                 ^~~~~

Restore the missing unistd.h include.

Fixes: 115b06a00875 ("tools/rtla: Consolidate nr_cpus usage across all tools")
Signed-off-by: Andreas Ziegler <br025@umbiko.net>
Link: https://lore.kernel.org/r/20260624033342.129970-1-br025@umbiko.net
[ capital letter in patch subject, remove EOL after Fixes ]
Signed-off-by: Tomas Glozar <tglozar@redhat.com>