lttng-ust: update to 2.15.0

lttng-ust: update to 2.15.0

Update from 2.13.9 to 2.15.0 (skips 2.14.x stable series).

Highlights of the 2.14/2.15 series:
- C++ header compatibility improvements (tracepoint API usable from C++)
- liblttng-ust-tracepoint split into its own shared library
- Add liblttng-ust-fd, liblttng-ust-fork helper libraries
- Ring buffer API cleanup and modernisation
- Drop internal libcompat layer

Patch update:
- 100-no-tests.patch: adjust hunk offset from line 7 to line 9; the
  tests/ entry in SUBDIRS moved down two lines in Makefile.am

Reference: https://lttng.org/files/lttng-ust/

Signed-off-by: Alexandru Ardelean <redacted>