allow building with GRASS and without PostgreSQL

[dhdeangelis]

Added changes suggested by @m-kuhn in #60038 (comment)

in: src/providers/grass/CMakeLists.txt:135 to

target_link_libraries(qgisgrass${GRASS_BUILD_VERSION} ${GRASS_TARGET_LINK_LIBRARIES${GRASS_BUILD_VERSION}})
if(WITH_POSTGRESQL)
  target_link_libraries(qgisgrass${GRASS_BUILD_VERSION} PostgreSQL::PostgreSQL)
endif()

This solves an issue encountered when building QGIS with GRASS but without PostgreSQL, when PostgreSQL::PostgreSQL could not be found.

Tested locally, works OK.

[github-actions]

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