Fix build wrt. missing CFLAGS constituents
When libosmo-netif and/or libosmo-abis are installed in distinct
prefixes, the build failed with non-found headers.
diff --git a/openbsc/tests/gprs/Makefile.am b/openbsc/tests/gprs/Makefile.am
index 616042b..633c362 100644
--- a/openbsc/tests/gprs/Makefile.am
+++ b/openbsc/tests/gprs/Makefile.am
@@ -1,5 +1,5 @@
AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include
-AM_CFLAGS=-Wall -ggdb3 $(LIBOSMOCORE_CFLAGS) $(LIBOSMOGSM_CFLAGS)
+AM_CFLAGS=-Wall -ggdb3 $(LIBOSMOCORE_CFLAGS) $(LIBOSMOGSM_CFLAGS) $(LIBOSMOABIS_CFLAGS)
EXTRA_DIST = gprs_test.ok