Make dependencies broken

[michalfita]

The 70d4c1c for #85 introduced problems with Make targets where generation of extensions loader isn't dependencies for all targets invoking cargo, where root crate refers to the generated dependency.

Symptom:

error: failed to load manifest for workspace member `/home/manveru/projects/kerla/kernel`

Caused by:
  failed to load manifest for dependency `kerla_kexts_loader`

Caused by:
  failed to read `/home/manveru/projects/kerla/build/kexts_loader/Cargo.toml`

Caused by:
  No such file or directory (os error 2)
make: *** [Makefile:142: check] Error 101