rgw: cmake: remove useless civetweb include path side effect.

rgw: cmake: remove useless civetweb include path side effect.

For 'target_include_directories" for the cmake object library
'civetweb_common_objs', change from PUBLIC to PRIVATE.  This doesn't
break anything, so it wasn't doing anything useful.  If it has it
any effect, it would be to cause everything that linked against this
"library" to also use the indictated include path.  Which would be great
except everything in ceph wants to include "civetweb/civetweb.h" and
not "civetweb.h".  We already make separate arrangements elsewhere for
that to work.  Additionally, static object libraries in cmake aren't
really libraries, so I'm not entirely sure this even does anything.
So: making this public is not useful, and could be harmful.  Making it
private makes this only take effect for building civetweb.c itself,
exactly the effect we we require, and no more.

Signed-off-by: Marcus Watts <mwatts@redhat.com>
(cherry picked from commit 8308a13b0257c9460fd2a721c20b0c37cb9e7c57)