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/src/libtrau/Makefile.am b/openbsc/src/libtrau/Makefile.am
index 0c8cf17..0fe3a53 100644
--- a/openbsc/src/libtrau/Makefile.am
+++ b/openbsc/src/libtrau/Makefile.am
@@ -1,5 +1,5 @@
AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
-AM_CFLAGS=-Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOVTY_CFLAGS) $(LIBOSMOABIS_CFLAGS) $(COVERAGE_CFLAGS)
+AM_CFLAGS=-Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOVTY_CFLAGS) $(LIBOSMOABIS_CFLAGS) $(LIBOSMONETIF_CFLAGS) $(COVERAGE_CFLAGS)
AM_LDFLAGS = $(LIBOSMOCORE_LIBS) $(LIBOSMOGSM_LIBS) $(LIBOSMOABIS_LIBS) $(COVERAGE_LDFLAGS)
noinst_LIBRARIES = libtrau.a