cmake: Fix stale codegen when mpgen binary changes #316

Add MPGEN_BINARY to DEPENDS so target_capnp_sources() also rebuilds when mpgen binary changes. This prevents stale code gen when mpgen binary changes but capnp files don't.

I noticed this after seeing IWYU errors while building the examples, which were caused by stale generated code that no longer matched what the current mpgen produces.

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ACK eea9c64f6e1682f7ad7c4a0e7ffbcc8c61619b21.

From CMake docs:

List target names with the DEPENDS option to add ... file-level dependencies.

ACK eea9c64f6e1682f7ad7c4a0e7ffbcc8c61619b21

I noticed this after seeing IWYU errors while building the examples, which was caused by stale generated code that no longer matched what the current mpgen produces.

Nice catch! I experienced this too, but I ignored it thinking there was something wrong with my machine.

Code review ACK eea9c64f6e1682f7ad7c4a0e7ffbcc8c61619b21. Nice fix which should make testing generator changes more convenient.