spirv-headers : Add file to restrict to commit

spirv-headers : Add file to restrict to commit

Restrict spirv-headers to a specific commit (just as we do to
spirv-tools and glslang) by creating a new file
(spirv-headers_revision) to track the commit we want to
syncrhonize to.  Also update the update_external_sources scripts
to use it.  This will make sure that we don't run into the
scenario where spirv-tools and spirv-headers are incompatible.

Change-Id: Ia4fdd921a5c1c500d7f751db08b6998b3d704f2d