soci: fix linking against dependencies with conanv1

[Cogitri]

soci searches for POSTGRESQL_LIBRARIES etc. instead of what conan defines (PostgreSQL_LIBRARIES).

fixes #11628

---

• I've read the contributing guidelines.
• I've used a recent Conan client version close to the currently deployed.
• [] I've tried at least one configuration locally with the conan-center hook activated.

[github-actions]

Hooks produced the following warnings for commit 59873f3

soci/4.0.3

post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libsoci_core.so' links to system library 'dl' but it is not in cpp_info.system_libs.
post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libsoci_core.so' links to system library 'pthread' but it is not in cpp_info.system_libs.
post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libsoci_core.so' links to system library 'm' but it is not in cpp_info.system_libs.
post_package(): WARN: [APPLE RELOCATABLE SHARED LIBS (KB-H077)] install_name dir of these shared libs is not @rpath: libsoci_core.4.0.3.dylib, libsoci_core.4.0.dylib, libsoci_core.dylib

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 4 (abcdb8c8b69bd0cd4dcc82f2084ba509dd33fa85):

• soci/4.0.3@:
  All packages built successfully! (All logs)

---

Conan v2 pipeline ✔️

Note: Conan v2 builds may be required once they are on the v2 ready list

All green in build 4 (abcdb8c8b69bd0cd4dcc82f2084ba509dd33fa85):

• soci/4.0.3@:
  All packages built successfully! (All logs)

[github-actions]

Hooks produced the following warnings for commit abcdb8c

soci/4.0.3

post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libsoci_core.so' links to system library 'dl' but it is not in cpp_info.system_libs.
post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libsoci_core.so' links to system library 'pthread' but it is not in cpp_info.system_libs.
post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libsoci_core.so' links to system library 'm' but it is not in cpp_info.system_libs.