kselftest: Support old perl versions

On an old perl such as v5.10.1, `kselftest/prefix.pl` gives below error
message:

    Can't locate object method "autoflush" via package "IO::Handle" at kselftest/prefix.pl line 10.

This commit fixes the error by explicitly specifying the use of the
`IO::Handle` package.
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/kselftest/prefix.pl

@@ -3,6 +3,7 @@
 # Prefix all lines with "# ", unbuffered. Command being piped in may need
 # to have unbuffering forced with "stdbuf -i0 -o0 -e0 $cmd".
 use strict;
+use IO::Handle;
 
 binmode STDIN;
 binmode STDOUT;