Log stderr of secilc

This makes init log stderr of secilc invoked to compile SELinux
policy. Having an explanation for why secilc failed is very useful for
debugging boot issues.

Test: Device with PRODUCT_FULL_TREBLE boots up just fine
Test: Modified init.cpp to reference non-existent .cil file on a device
      with PRODUCT_FULL_TREBLE and confirmed that dmesg now contains the
      error message from secilc saying that the file was not found.
Bug: 31363362

Change-Id: I6a3b3576daf0d6fd09e2c79bc43ae63850f44a00