tools/expat: bump to 2.7.1 to fix several CVEs

tools/expat: bump to 2.7.1 to fix several CVEs

Addresses CVE-2024-8176 and CVE-2024-50602.

Changelog: https://github.com/libexpat/libexpat/blob/R_2_7_1/expat/Changes
Fixes: https://github.com/openwrt/packages/issues/26255
Fixes: https://github.com/advisories/GHSA-9hcv-xw76-m4h6
Fixes: https://github.com/advisories/GHSA-79wf-qgrg-2p6c
Signed-off-by: George Sapkin <george@sapk.in>
Link: https://github.com/openwrt/openwrt/pull/18421
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 14a88ba520b44cf22216d1819d936cea1ec509a9)
Link: https://github.com/openwrt/openwrt/pull/19079
Signed-off-by: Robert Marko <robimarko@gmail.com>