The extra `../` only works if the build directory is an immediate child of the
source directory. In all the other configurations this breaks the build.

Dropping the extra `../` makes use of the `-I${SOURCE_DIR}` provided by meson.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>

and in case it's missing, also #define some things to make compilation work,
since this is only available in a pipewire branch right now

This will solve a possible race condition in the unit test when destroying the
autotestsrc proxy while its node_event_info is being called.

the global is stored internally and the returned ref is only useful
in the WpProxy code, not in the registry_global() event

it doesn't matter, because we don't use it, but for the sake of readability

some code is expecting things to happen in that order...