ktest: Use make -s kernelrelease

The previous tail -1 broke with commit 7ff525712a ("kbuild: fake the
"Entering directory ..." message more simply")

Link: http://lkml.kernel.org/r/20141022194408.GA20989@pobox.suse.cz

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

tools/testing/ktest/ktest.pl

@@ -2005,7 +2005,7 @@ sub get_version {
     # get the release name
     return if ($have_version);
     doprint "$make kernelrelease ... ";
-    $version = `$make kernelrelease | tail -1`;
+    $version = `$make -s kernelrelease`;
     chomp($version);
     doprint "$version\n";
     $have_version = 1;