span-lite: backport fix to avoid -Wundef error in C++20

span-lite: backport fix to avoid -Wundef error in C++20

When building babeltrace as C++20, I get:

      CXX      clock-correlation-validator/clock-correlation-validator.lo
    In file included from /home/smarchi/src/babeltrace/src/cpp-common/bt2s/span.hpp:18,
                     from /home/smarchi/src/babeltrace/src/cpp-common/bt2c/logging.hpp:25,
                     from /home/smarchi/src/babeltrace/src/cpp-common/bt2/component-class-dev.hpp:15,
                     from /home/smarchi/src/babeltrace/src/cpp-common/bt2/component-class.hpp:15,
                     from /home/smarchi/src/babeltrace/src/cpp-common/bt2/error.hpp:20,
                     from /home/smarchi/src/babeltrace/src/cpp-common/bt2/wrap.hpp:15,
                     from /home/smarchi/src/babeltrace/src/clock-correlation-validator/clock-correlation-validator.cpp:9:
    /home/smarchi/src/babeltrace/src/cpp-common/vendor/span-lite/span.hpp:43:33: error: "span_HAVE_STRUCT_BINDING" is not defined, evaluates to 0 [-Werror=undef]
       43 | #define span_HAVE( feature )  ( span_HAVE_##feature )
          |                                 ^~~~~~~~~~
    /home/smarchi/src/babeltrace/src/cpp-common/vendor/span-lite/span.hpp:1873:5: note: in expansion of macro 'span_HAVE'
     1873 | #if span_HAVE( STRUCT_BINDING )
          |     ^~~~~~~~~

Backport commit b8921715cd71 ("Guard section for tuple interface to only
use with nonstd::span; fixes #84 (thanks @simark)") from the upstream
repo [1]

[1] https://github.com/martinmoene/span-lite/commit/b8921715cd71997bdab120a8ced526b78d300196

Change-Id: I84b21f8157f7bf44e02e67789cfa1fee2a38da05
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12525