tests: make functions static in test_utils_compat_poll.c

tests: make functions static in test_utils_compat_poll.c

Fixes:

      CC       test_utils_compat_poll.o
    /home/smarchi/src/lttng-tools/tests/unit/test_utils_compat_poll.c:74:6: error: no previous declaration for ‘test_alloc’ [-Werror=missing-declarations]
     void test_alloc(void)
          ^~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_utils_compat_poll.c:95:6: error: no previous declaration for ‘test_add_del’ [-Werror=missing-declarations]
     void test_add_del(void)
          ^~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_utils_compat_poll.c:133:6: error: no previous declaration for ‘test_mod_wait’ [-Werror=missing-declarations]
     void test_mod_wait(void)
          ^~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_utils_compat_poll.c:189:6: error: no previous declaration for ‘test_func_def’ [-Werror=missing-declarations]
     void test_func_def(void)
          ^~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I1feeed8ee3f76783e22ffedd5292a06deee3e233
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>