Use "-s" instead of "--quiet" to resolve non-fatal build error on FreeBSD.

The --quiet argument is not available on all operating systems.  Use -s
instead to match the rest of the Makefile uses.  This fixes a non-fatal
error seen on FreeBSD.
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NSean Bruno <sbruno@freebsd.org>
Message-Id: <20160614180734.8782-1-sbruno@freebsd.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Makefile

@@ -185,7 +185,7 @@ qemu-version.h: FORCE
 				printf '""\n'; \
 			fi; \
 		fi) > $@.tmp)
-	$(call quiet-command, cmp --quiet $@ $@.tmp || mv $@.tmp $@)
+	$(call quiet-command, cmp -s $@ $@.tmp || mv $@.tmp $@)
 
 config-host.h: config-host.h-timestamp
 config-host.h-timestamp: config-host.mak