compat: remove always true assertion in compat-poll.c

compat: remove always true assertion in compat-poll.c

Fix:

      CC       compat-poll.lo
    In file included from /home/smarchi/src/lttng-tools/src/common/compat/poll.h:21:0,
                     from /home/smarchi/src/lttng-tools/src/common/compat/compat-poll.c:31:
    /home/smarchi/src/lttng-tools/src/common/compat/compat-poll.c: In function ‘compat_poll_wait’:
    /home/smarchi/src/lttng-tools/src/common/compat/compat-poll.c:317:31: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
      assert(events->current.nb_fd >= 0);
                                   ^

Change-Id: Ie2b43e64d293d4eadb82db9755dfb64751a1d935
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>