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
diff --git a/.gitignore b/.gitignore
index 3480380..70193ff 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 *.o
 *.lo
 *.a
+*.la
 .deps
 Makefile
 Makefile.in