BUILD: compiler: fixed a missing test on  defined(__GNUC__)

This one could theoretically trigger -Wundef on non-gcc compatible
compilers if DEBUG_USE_ABORT is not set.

(cherry picked from commit 8ac6597cbed48d8d727eb59427713099f815ccbb)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed