libpam: re-add missing libgen include

libpam: re-add missing libgen include

It was added by original commit for CVE-2025-6020-01.patch
https://github.com/linux-pam/linux-pam/commit/475bd60c552b98c7eddb3270b0b4196847c0072e#diff-05f443e6acbe32a148a45648148739bf6f02f13acc5c20c6037bf933223d4d77
but removed here in the rebase, causing:

../../../Linux-PAM-1.5.3/modules/pam_namespace/pam_namespace.c:326:11: error: call to undeclared function 'dirname'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  326 |         parent = dirname(buf);
      |                  ^
../../../Linux-PAM-1.5.3/modules/pam_namespace/pam_namespace.c:326:9: error: incompatible integer to pointer conversion assigning to 'char*' from 'int' [-Wint-conversion]
  326 |         parent = dirname(buf);
      |                ^ ~~~~~~~~~~~~

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>