Fix: liblttng-ctl: poll compatibility symbols inadvertently exported

abidiff reports that the following poll/epoll compatibility layer
symbols were inadventently exported by liblttng-ctl:

'function int compat_epoll_add(compat_epoll_event*, int, uint32_t)'    {compat_epoll_add}
'function int compat_epoll_create(compat_epoll_event*, int, int)'    {compat_epoll_create}
'function int compat_epoll_del(compat_epoll_event*, int)'    {compat_epoll_del}
'function int compat_epoll_mod(compat_epoll_event*, int, uint32_t)'    {compat_epoll_mod}
'function int compat_epoll_set_max_size()'    {compat_epoll_set_max_size}
'function int compat_epoll_wait(compat_epoll_event*, int, bool)'    {compat_epoll_wait}

Those functions and their 'poll' equivalent are marked as
LTTNG_HIDDEN.

The poll_max_size variable is also made static and removed from the
compatibility header since it is never used apart from within the
implementation files.

'unsigned int poll_max_size'    {poll_max_size}

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>