auth-pam.c: add missing include limits.h

auth-pam.c: add missing include limits.h

On most systems limits.h is pulled in by some other header and thus no
error is ever triggered, but it's possible to find the right environment
which lackis this and prevents compiling auth-pam.c (possibly when using
LibreSSL).

Include the header explicitly as it includes the definition of PATH_MAX.

(note that this bug is fixed in Gentoo since 2020 by including a custom
patch, but apparently the issue was never reported upstream)

Reported-by: Michelangelo Scopelliti <kernelpanic@gmx.com>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220421131909.32053-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24136.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 0fed64a91d894b46105bf7e8b16edea4d90ab70c)