fs_config reads headers from snapshots, if available

fs_config requires the information from the header files in
system/core and bionic/libc. To build the vendor side fs_config,
use the header files in the vendor snashot if the required version
exists.

Bug: 187222756
Test: check ninja depedency with and without snasphots.
Change-Id: Ibf96eab4d9a129745be1a19b2aa2e4c8f57cf6bf