FTBFS with -Werror=implicit-function-declaration

As part of time_t work, -Werror=implicit-function-declaration is being set on armhf, but that causes the following FTBFS on any arch with the same -Werror=implicit-function-declaration setting:

https://launchpadlibrarian.net/716768885/buildlog_ubuntu-noble-armhf.pam_1.5.3-5ubuntu1_BUILDING.txt.gz

In file included from obscure.c:42:
obscure.c: In function ‘password_check’:
support.h:160:9: error: implicit declaration of function ‘pam_overwrite_string’; did you mean ‘_pam_overwrite_n’? [-Werror=implicit-function-declaration]
  160 | pam_overwrite_string(xx); \
      | ^~~~~~~~~~~~~~~~~~~~
obscure.c:151:9: note: in expansion of macro ‘_pam_delete’
  151 | _pam_delete(newmono);
      | ^~~~~~~~~~~