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: NEd Maste <emaste@freebsd.org>
Acked-by: NAndi Kleen <ak@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: http://lore.kernel.org/lkml/20200528170858.48457-1-emaste@freefall.freebsd.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/pmu-events/jsmn.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: MIT */
 #ifndef __JSMN_H_
 #define __JSMN_H_