unit_tests/plugins/auth-pam: fix stdint.h related build error on fedora 42

unit_tests/plugins/auth-pam: fix stdint.h related build error on fedora 42

add <stdint.h> to test_search_and_replace.c to fix build error on
fedora 42 / arm64 ("error: uintptr_t undeclared")

Change-Id: I2ab13767b5aa858e024b45be3d161bf6090de763
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20250728104234.29797-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32384.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>