Cleanup compile warning

ustctl.c: In function 'ustctl_init':
ustctl.c:1772:2: warning: implicit declaration of function
'lttng_ring_buffer_client_overwrite_rt_init'
[-Wimplicit-function-declaration]
ustctl.c:1774:2: warning: implicit declaration of function
'lttng_ring_buffer_client_discard_rt_init'
[-Wimplicit-function-declaration]
ustctl.c: In function 'ustctl_exit':
ustctl.c:1781:2: warning: implicit declaration of function
'lttng_ring_buffer_client_discard_rt_exit'
[-Wimplicit-function-declaration]
ustctl.c:1783:2: warning: implicit declaration of function
'lttng_ring_buffer_client_overwrite_rt_exit'
[-Wimplicit-function-declaration]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>