Relicense src/compat-smp.h to MIT

Relicense src/compat-smp.h to MIT

Relicense the code in src/compat-smp.h from LGPLv2.1 to MIT to match the
copies in the following projects:

- lttng-ust
- libside
- librseq

This code is entirely authored by EfficiOS.

This relicensing initially appeared in the lttng-ust project after
the code was imported into liburcu:

  commit 4159f02937a2740abd7f5b113f376b198a86bc71 (test-struct-tls)
  Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  Date:   Tue Oct 25 12:32:12 2022 -0400

      Relicense common/smp.c common/smp.h to MIT

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ib7edb6c18fd3c004503b9c023ba4e280241ede14