cpp-common/bt2c/fmt.hpp: use `wise_enum::string_type` in `EnableIfIsWiseEnum` definition

cpp-common/bt2c/fmt.hpp: use `wise_enum::string_type` in `EnableIfIsWiseEnum` definition

wise_enum uses `const char *` as the string type for C++11/14 and
`std::string_view` for C++ >= 17.  Change `EnableIfIsWiseEnum` to use
`wise_enum::string_type` instead of a hard-coded `const char *`, to make
compilation in C++17 possible.

Change-Id: Ie179e1f5585128950ecdc095f7b136d93ea39e41
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12497
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>