meson: Add pg_regress_ecpg to ecpg test dependencies

meson: Add pg_regress_ecpg to ecpg test dependencies

This is required to ensure correct test dependencies, previously
pg_regress_ecpg would not necessarily be built.

The missing dependency was, e.g., visible when doing
  ninja clean && ninja meson-test-prereq && meson test --no-rebuild --suite setup --suite ecpg

Apply this to all branches with meson support, as part of an effort to fix
incorrect test dependencies that can lead to test failures.

Discussion: https://postgr.es/m/CAGECzQSvM3iSDmjF+=Kof5an6jN8UbkP_4cKKT9w6GZavmb5yQ@mail.gmail.com
Discussion: https://postgr.es/m/bdba588f-69a9-4f3e-9b95-62d07210a32e@eisentraut.org
Backpatch: 16-, where meson support was added