dbus/dbus-macros-internal, test/test-platform-mutex.c: Use upper case for SPDX license operator
From: @rhabacker
https://spdx.github.io/spdx-spec/SPDX-license-expressions/ says that using upper-case operators is canonical.
This is really a syntax fix more than a licensing clarification.
Previously part of a larger commit in !304, and then part of !311 (merged). Separated out here so that we can merge SPDX licensing a bit at a time until everything is resolved.