logging.h: fix backwards compat broken by recent commit

Commit 812ba6dc63a75c39678dd3fe652768e76bf63183
"logging: centrally define ctx and filter indexes"
Removed definitions, which causes older e.g. openbsc and osmo-pcu code trees to
fail to build against a newer libosmocore. Re-introduce the legacy definitions
to redirect to the new ones and re-establish backwards compatibility.

The GPRS_* constants used to be defined in gprs_msgb.h, but since that header
also includes logging.h, rather place the legacy shims in logging.h next to the
other ones.

Change-Id: I455bb1bb474d758af0fd5b6397f7e57260ad739d
1 file changed