lttng: build the tracepoint provider lib from .c files in repo

lttng: build the tracepoint provider lib from .c files in repo

we should not have `TRACEPOINT_DEFINE` deinfed in both the application
we are tracing and the tracing provider library.

* tracing/*.c: reference the header file with path
* README.md: add instructions to add .c file
* CMakeLists.c:
  - compile the .c file in repo, not the one generated by
    lttng-gen-tp
  - no need to mark the generated header file as GENEREATED,
    they are marked so by default (they are listed in OUTPUT)
  - do not generate .c file anymore, they are not used.

Signed-off-by: Kefu Chai <kchai@redhat.com>