ktest: Minor cleanup with uninitialized variable $build_options

Signed-off-by: John 'Warthog9' Hawley (VMware) <[e-mail]>
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>

tools/testing/ktest/ktest.pl

···
     # Run old config regardless, to enforce min configurations
     make_oldconfig;
 
+    if (not defined($build_options)){
+	$build_options = "";
+    }
     my $build_ret = run_command "$make $build_options", $buildlog;
 
     if (defined($post_build)) {