lttng-sessiond: do not call ustctl_register_done()

When lttng-ust support is not enabled, compilation might fail with:
lttng-tools.git/lttng-sessiond/main.c:1167: undefined reference to
`ustctl_register_done'

Actually when HAVE_LIBLTTNG_UST_CTL is not defined, the function call is
normally optimized out by the compiler because ust_app_register() is a
static inline alway returning a negative value, leaving the call within
unreachable code.

Depending on the compiler version and optimization flags, this may
however not always happen, leading to the above error.

Therefore replace ustctl_register_done() with ustapp_register_done(),
which calls the original function when lttng-ust is enabled, and returns
an error otherwise (it is unreachable code anyway).

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>