Makefile.am: Use AM_CPPFLAGS

Since automake 1.13 INCLUDES is depricates and causes a warning
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 11b605f..fea78fc 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,4 +1,4 @@
-INCLUDES = $(all_includes) -I$(top_srcdir)/include
+AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include
 AM_CFLAGS=-Wall -g $(LIBOSMOCORE_CFLAGS) $(LIBOSMOGSM_CFLAGS) $(COVERAGE_CFLAGS)
 AM_LDFLAGS = $(COVERAGE_LDFLAGS)