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/osmo-bsc_mgcp/Makefile.am b/openbsc/src/osmo-bsc_mgcp/Makefile.am
index fba76b4..c62131b 100644
--- a/openbsc/src/osmo-bsc_mgcp/Makefile.am
+++ b/openbsc/src/osmo-bsc_mgcp/Makefile.am
@@ -1,6 +1,7 @@
 AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
 AM_CFLAGS=-Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOGSM_CFLAGS) \
-	$(LIBOSMOVTY_CFLAGS) $(LIBOSMOABIS_CFLAGS) $(COVERAGE_CFLAGS)
+	$(LIBOSMOVTY_CFLAGS) $(LIBOSMOABIS_CFLAGS) \
+	$(LIBOSMONETIF_CFLAGS) $(COVERAGE_CFLAGS)
 
 bin_PROGRAMS = osmo-bsc_mgcp