libpam: re-add missing libgen include

It was added by original commit for CVE-2025-6020-01.patch
475bd60c55 (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);
      |                ^ ~~~~~~~~~~~~

Backport 6d88a28ac7b6ff61808eb46e5c85dabd17c77f2e from scarthgap.
It's reproducible with clang-18 from kirkstone-clang18 branch of
meta-clang.

(From OE-Core rev: 6888cb09ad069cd937ac4498640fdd5bed2e7a51)

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Paul Barker <paul@pbarker.dev>

meta/recipes-extended/pam/libpam/CVE-2025-6020-01.patch

@@ -1528,7 +1528,7 @@ diff --git a/modules/pam_namespace/pam_namespace.h b/modules/pam_namespace/pam_n
 index b51f284..abd570d 100644
 --- a/modules/pam_namespace/pam_namespace.h
 +++ b/modules/pam_namespace/pam_namespace.h
-@@ -44,21 +44,16 @@
+@@ -44,21 +44,17 @@
  #include <stdlib.h>
  #include <errno.h>
  #include <syslog.h>
@@ -1542,7 +1542,7 @@ index b51f284..abd570d 100644
 -#include <sys/resource.h>
  #include <sys/mount.h>
  #include <sys/wait.h>
--#include <libgen.h>
+ #include <libgen.h>
  #include <fcntl.h>
  #include <sched.h>
  #include <glob.h>