bp2build: build //system/core/libprocessgroup:libprocessgroup_headers

This adds //system/core/libprocessgroup to the denylist, but
non-recursively, because there are other modules in its subpackages that
don't build yet, so CI's bp2build-incremental's `//system/...` build can keep
working.

Fixes: 186826841
Test: bp2build-incremental //system/...
Change-Id: I4e778737414364964de6328d7420512da224809c