Make fmtlib_headers available to CTS. am: d0da765300 am: 6327cfc490 am: fb95463406 am: 1ec478a7f7

Original change: https://android-review.googlesource.com/c/platform/external/fmtlib/+/2725756

Change-Id: I3c78213c174e00e46e57500a8e3c50403595b4ed
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>