Use correct includes for mixed build gen headers

Previously, genrules which were used as generated_headers only
propagated the old Soong gen directory as an include path, even if the
genrule was managed by Bazel. With this chance, the gen directory of the
Baze output files are used in mixed-build mode.

Test: USE_BAZEL_ANALYSIS=1 m libc
Test: m libc
Change-Id: I3913c4e5b0354421ba1fe43d1f6ef4d1e729a058