Makefile.am: Use AM_CPPFLAGS

Since automake 1.13 INCLUDES is depricates and causes a warning
diff --git a/openbsc/src/osmo-nitb/Makefile.am b/openbsc/src/osmo-nitb/Makefile.am
index dc69232..ce2f25d 100644
--- a/openbsc/src/osmo-nitb/Makefile.am
+++ b/openbsc/src/osmo-nitb/Makefile.am
@@ -1,4 +1,4 @@
-INCLUDES = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
+AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
 AM_CFLAGS=-Wall	$(COVERAGE_CFLAGS)			\
   	$(LIBOSMOCORE_CFLAGS) $(LIBOSMOGSM_CFLAGS) $(LIBOSMOVTY_CFLAGS) \
 	$(LIBOSMOABIS_CFLAGS) $(LIBSMPP34_CFLAGS)