Fix -Wduplicated-cond warnings

Fixes this:

      CC       libctf_parser_la-parser.lo
    /home/smarchi/src/babeltrace/src/plugins/ctf/common/metadata/parser.y: In function ‘yyparse’:
    /home/smarchi/src/babeltrace/src/plugins/ctf/common/metadata/parser.y:1313:50: error: duplicated ‘if’ condition [-Werror=duplicated-cond]
        } else if ($$->u.unary_expression.type == UNARY_UNSIGNED_CONSTANT) {
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/babeltrace/src/plugins/ctf/common/metadata/parser.y:1309:43: note: previously used here
        if ($$->u.unary_expression.type == UNARY_UNSIGNED_CONSTANT) {
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

I am pretty sure that this line should use UNARY_SIGNED_CONSTANT.

Change-Id: Icb17d79422ed78a214fb88a3a0787fcd760822d6
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2260
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>