textproc/expat2: security update 2.7.3 -> 2.7.4

ChangeLog: https://github.com/libexpat/libexpat/compare/R_2_7_3...R_2_7_4
PR: 293033
Approved by: fluffy (on behalf of ports-secteam)

(cherry picked from commit 94e627043b4dbcd3094d8acd95cd98f277671401)