cmake: Exclude generated sources from translation

[hebasto]

This PR fixes an error encountered when building the translate target:

$ gmake -j $(nproc) -C depends MULTIPROCESS=1
$ cmake -G "Unix Makefiles" --preset dev-mode --toolchain depends/x86_64-pc-linux-gnu/toolchain.cmake -DWITH_USDT=OFF
$ cmake --build build_dev_mode -t translate
gmake[3]: *** No rule to make target 'src/test/ipc_test.capnp.c++', needed by 'src/qt/CMakeFiles/translate'. Stop.
gmake[2]: *** [CMakeFiles/Makefile2:1646: src/qt/CMakeFiles/translate.dir/all] Error 2
gmake[1]: *** [CMakeFiles/Makefile2:1653: src/qt/CMakeFiles/translate.dir/rule] Error 2
gmake: *** [Makefile:699: translate] Error 2

The previous attempt to address this issue worked only with Ninja generators and has been reverted.

Essentially, this PR modifies the translate target so that it ignores generated sources rather than attempting to update them.

At present, multiprocess-specific sources do not contain any translatable strings. Nonetheless, it is prudent to maintain a general approach.