fix build if depenedencies are in distinct directories
diff --git a/src/hnbap/Makefile.am b/src/hnbap/Makefile.am
index a9e8163..e152587 100644
--- a/src/hnbap/Makefile.am
+++ b/src/hnbap/Makefile.am
@@ -232,7 +232,7 @@
 	PrivateMessage.h	\
 	IE.h
 
-AM_CFLAGS = $(ASN1C_CFLAGS) -I.
+AM_CFLAGS = $(ASN1C_CFLAGS) $(OSMOCORE_CFLAGS) -I.
 
 noinst_LIBRARIES=libosmo-asn1-hnbap.a
 libosmo_asn1_hnbap_a_SOURCES=$(ASN_MODULE_SOURCES)