fortify-headers: fix build error when _REDIR_TIME64 is not defined

fortify-headers: fix build error when _REDIR_TIME64 is not defined

some targets do not define the _REDIR_TIME64 macro resulting in a
build error regression since ddfe5678a448ac8875e94f2fb4ddca67416fa14a
fix by checking if the macro is defined

[1] https://github.com/openwrt/openwrt/issues/12587

Fixes: #12587
Fixes: ddfe5678a448 ("fortify-headers: fix inconsistent time_t version of ppoll")
Signed-off-by: Georgi Valkov <gvalkov@gmail.com>
(cherry picked from commit 9145c4fbcb655b2f56c76012da85cb203fdbc72b)