auth-pam: fix discards 'const' qualifier from pointer target type

auth-pam: fix discards 'const' qualifier from pointer target type

strstr now returns const char*.

Change-Id: I632368451923116e0a169ddb5b6e86a8f8486afc
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1543
Message-Id: <20260218214712.27119-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg35728.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit eeaedd13628e27225c8893c2d8bdf5381fc33ba9)