Do not glob headers in out/

Currently, it skips it for files in out/soong/. This was a valid
assumption since all intermediate artifacts in soong are placed under
out/soong, and soong modules cannot depend on make modules.

However, this might change in multitree, since the headers provided by
build orchestrator will be under out/api_surfaces (next CL in this stack). The good side effect
of this is that we do not retrigger a rebuild of soong/build.ninja if a
new API file (.h/.map.txt) is added to this directory (ctx.GlobWithDeps
adds the files to the deps of out/soong/build.ninja).

Test: TH
Change-Id: I3c3722390d04a50d793b1646df00cdf7acb12b49