BUILD/MINOR: regex: avoid a build warning on USE_PCRE2 with -Wundef

regex-t emits a warning on #elif USE_PCRE2 when built with -Wundef,
let's just fix it. This was reported in GH issue #1369.

(cherry picked from commit a01f8ce2d4cf54d1d2b210a8bf2b0437ed15cdb9)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed