build_tools/make_version: Fix sed(1) syntax compatibility with NetBSD

Fix the sed(1) invocation used to process git-svn-id not to use "\s"
that is a GNU-ism and is not supported by NetBSD sed.  As a result,
this call did not work properly and make_version did output the full
git-svn-id line rather than the revision.

ASTERISK-29852

Change-Id: Ie4b406e2748920643446851a0a252a4ca7245772