src/gprs/Makefile.am: Move build of shared .c files to an internal lib

Build files shared by osmo-sgsn, osmo-gbpy and osmo-gtphub into a .la
library, so we can later split each application into its own subdir and
clearly identify what's used by who.

Due to a dependency error with .Po files,  I cannot depend on the specific
.o files directly in LDADD for each binary, but it works fine on follow up
commits when binaries are splitted into different makefiles, so it will be
done later.

Change-Id: Ib7665c530c086a5f3135c395bb8bf19ed4a882b6
2 files changed