Fix: build failures when CONFIG_UPROBES is absent

Fix: build failures when CONFIG_UPROBES is absent

Problems
========
- There is a typo in the struct name of the parameters of stub version
  of the lttng_uprobes_add_callsite function,
- We are building the lttng-uprobes.o object file even when
  CONFIG_UPROBES is absent.

Both of these are causing build errors.

Fixes
=====
- Replace struct lttng_kernel_callsite_uprobe by struct
  lttng_kernel_event_callsite,
- Only add the lttng-uprobes.o object file to the needed artefacts if
  CONFIG_UPROBES is present.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>