Enable exporting the non-uapi scsi headers.

Callers are supposed to #include <scsi/sg.h> but if we tell soong to add
bionic/libc/kernel/android/ to the include path, the uapi headers in there
would be (unintentionally) accessible as either <linux/name.h> or
<uapi/linux/name.h>.

Bug: N/A (hit while upgrading strace to 4.17)
Test: builds
Change-Id: I8d47dd51da688c38f747a255d401dfb2c209c805