perf tools: Correct license on jsmn JSON parser

This header is part of the jsmn JSON parser, introduced in 867a979a

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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>