libutils: export system_headers

vendor + vendor_available modules built with BOARD_VNDK_VERSION :=
current aren't allowed to use global headers. Since libutils uses
system headers itself, it should export them as well.

Bug: 33241851
Test: building with BOARD_VNDK_VERSION := current produces many fewer
errors.

Change-Id: Icc7d2beb2a17da291434c6282eb1b9eb458f114c
Merged-In: I950821f1dae3ddc110db50869247913d5b1aa638