Skip default dependencies for header libraries

Header libraries don't compile anything, so they don't need the
default stl and libclang_rt dependencies.  Trimming the dependencies
simplifies using header libraries for LLNDK libraries, which need
extra variants that don't exist for the default dependencies.

Test: m checkbuild
Change-Id: Ib9864c3cc7048b2cc52acf304830777b65a51877