libgb: separate header files related to spec and implementation
like in libosmogsm, we separate between header files that are just
reflecting information in the respective specs, and header files that
related to our specific implementation.
diff --git a/openbsc/include/osmocom/gprs/Makefile.am b/openbsc/include/osmocom/gprs/Makefile.am
index 66b414f..d39592c 100644
--- a/openbsc/include/osmocom/gprs/Makefile.am
+++ b/openbsc/include/osmocom/gprs/Makefile.am
@@ -1,3 +1,5 @@
libgb_HEADERS = gprs_bssgp.h gprs_ns.h gprs_ns_frgre.h gprs_msgb.h
libgbdir = $(includedir)/osmocom/gprs
+
+SUBDIRS = protocol