cpp-common/bt2c: adjust syntax of operator"" to please g++ 4.8

cpp-common/bt2c: adjust syntax of operator"" to please g++ 4.8

Fixes:

      CXX      plugins/ctf/fs-src/data-stream-file.lo
    In file included from /home/smarchi/src/babeltrace/src/plugins/ctf/fs-src/data-stream-file.hpp:20:0,
                     from /home/smarchi/src/babeltrace/src/plugins/ctf/fs-src/data-stream-file.cpp:20:
    /home/smarchi/src/babeltrace/src/cpp-common/bt2c/data-len.hpp:179:23: error: missing space between '""' and suffix identifier
     static inline DataLen operator""_bits(const unsigned long long val) noexcept
                           ^

Change-Id: Iab7ab1273f13dd55735310f786a0bb4df63c967c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12794
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>