build: Generate a dependency for libs-r-t's generated headers
There's currently only one generated header, enums.h, but we could add more in future.
We need to make sure the test_utils static library depends on that generated header, otherwise it could be compiled before the header itself is generated, leading to intermittent and unreproducible build failures (for some reason scout i386 seems especially prone to this).
/cc @denittis