fix grass plugin build (followup d2078b00)

[jef-n]

No description provided.

[github-actions]

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(Built from commit 453ea56)

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(Built from commit 453ea56)

[m-kuhn]

@jef-n do you still have the build error around somewhere?

[jef-n]

@jef-n do you still have the build error around somewhere?

In file included from /usr/lib/grass84/include/grass/vect/digit.h:3,
                 from /usr/lib/grass84/include/grass/vector.h:4,
                 from /build/qgis-3.41.0+git20241231+834b94b0c01+99sid/src/providers/grass/qgsgrassvectormaplayer.h:38,
                 from /build/qgis-3.41.0+git20241231+834b94b0c01+99sid/src/providers/grass/qgsgrassvectormap.h:26,
                 from /build/qgis-3.41.0+git20241231+834b94b0c01+99sid/src/plugins/grass/qgsgrasseditrenderer.cpp:29:
/usr/lib/grass84/include/grass/vect/dig_structs.h:31:10: fatal error: libpq-fe.h: No such file or directory
   31 | #include <libpq-fe.h>
      |          ^~~~~~~~~~~~
compilation terminated.

[m-kuhn]

This should be added to a grass target and because we don't have that to the grass provider instead.
Does it help if you add PostgreSQL::PostgreSQL here https://github.com/qgis/QGIS/blob/master/src/providers/grass/CMakeLists.txt#L135 ?