lttng: Check for lttng/tracepoint.h when configuring
lttng: Add TRACEPOINT_INCLUDE_FILE definitions for old versions of LTTng
lttng: Move trace includes above assert include

Fixes errors that look like:
osd/PG.cc: In member function 'void PG::lock(bool)':
osd/PG.cc:226:421: error: base operand of '->' is not a pointer
osd/PG.cc:226:310: warning: unused variable '_dout_cct' [-Wunused-variable]
Merge pull request #2008 from adamcrume/wip-lttng-fixes

lttng fixes