libpmc: add MIT SPDX tag to header file

The jevents tool includes a copy of the jsmn json parser which is MIT
licensed. Upstream the MIT license appears in the jsmn.c source and a
standalone LICENSE file, but the latter is not included in the copy
contained in libpmc and the jsmn.h header carried no license information.
Add an SPDX tag to clarify the situation.