[PATCH v3] selftests/landlock:Fix two build issues

[kernel test robot]

Hi Hu,

kernel test robot noticed the following build errors:

[auto build test ERROR on shuah-kselftest/next]
[also build test ERROR on shuah-kselftest/fixes linus/master v6.7 next-20240112]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Hu-Yadi/selftests-landlock-Fix-two-build-issues/20240112-151805
base:   https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git next
patch link:    https://lore.kernel.org/r/20240112071245.669-1-hu.yadi%40h3c.com
patch subject: [PATCH v3] selftests/landlock:Fix two build issues
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240115/202401151147.T1s11iHJ-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp at intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202401151147.T1s11iHJ-lkp@intel.com/

All errors (new ones prefixed by >>):

>> net_test.c:25:14: error: static declaration of 'gettid' follows non-static declaration
      25 | static pid_t gettid(void)
         |              ^~~~~~
   In file included from /usr/include/unistd.h:1218,
                    from /usr/include/x86_64-linux-gnu/bits/sigstksz.h:24,
                    from /usr/include/signal.h:328,
                    from /usr/include/x86_64-linux-gnu/sys/wait.h:36,
                    from common.h:16,
                    from net_test.c:22:
   /usr/include/x86_64-linux-gnu/bits/unistd_ext.h:34:16: note: previous declaration of 'gettid' with type '__pid_t(void)' {aka 'int(void)'}
      34 | extern __pid_t gettid (void) __THROW;
         |                ^~~~~~

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