Silence -Wunused-but-set-variable error with clang

Silence -Wunused-but-set-variable error with clang

When building with clang-15, I get:

      CXX      libctf_parser_la-parser.lo
    parser.cpp:2920:9: error: variable 'yynerrs' set but not used [-Werror,-Wunused-but-set-variable]
        int yynerrs;
            ^

This is because it warns for something like this:

    int n;
    n = 0;
    ++n;

whereas previous versions do not.

This is generated code, so there's not much more we can do other than
silence the warning for the file.

Change-Id: If2cffe9042b743ffe4b4b23f751216f67b58e5fa
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8655
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>