perf tools: Correct license on jsmn JSON parser

This header is part of the jsmn JSON parser, introduced in 867a979a.
Correct the SPDX tag to indicate that it is under the MIT license.

Signed-off-by: Ed Maste <emaste@freebsd.org>
Acked-by: Andi Kleen <ak@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: http://lore.kernel.org/lkml/20200528170858.48457-1-emaste@freefall.freebsd.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>