lttng-tools: update to 2.15.0

lttng-tools: update to 2.15.0

Remove obsolete patches:
- 010-compat-off64_t-is-not-defined-by-musl.patch: the patched files
  (src/common/compat/compat-fcntl.c and src/common/compat/fcntl.h)
  no longer exist in 2.15.0
- 020-fix-lttng-tools-fails-to-compile-with-libxml2-2-14-0.patch: the
  encode_string() function was refactored in 2.15.0 to use
  xmlCharEncInFunc() instead of handler->input(), so the fix is no
  longer needed

Add musl compatibility fixes in Build/Prepare via sed:
- Remove :: global-namespace qualifier from TFD_CLOEXEC in timerfd.hpp;
  musl defines it as an octal literal so ::TFD_CLOEXEC is invalid C++
- Same fix for EPOLL_CLOEXEC in poller.cpp
- Relax static_assert in consumer.hpp from __cplusplus == 201103L to
  >= 201103L; SDK builds with C++17

Add missing +libstdcpp to DEPENDS (lttng-tools links libstdc++.so.6).

Signed-off-by: Alexandru Ardelean <redacted>