Speed up clean meson builds by ~25%

Building the generated ecpg preproc file can take a long time. You can
check how long using:

ninja -C build src/interfaces/ecpg/preproc/ecpg.p/meson-generated_.._preproc.c.o

This moves that file much closer to the top of our build order, so
building it can be pipelined much better with other files.

It improved clean build times on my machine (10 cores/20 threads) from ~40
seconds to ~30 seconds.

You can check improvements for yourself with:

ninja -C build clean && ninja -C build all