tests/oap: depend on libgtp

exclude logic copied from src/gprs/Makefile.am
diff --git a/openbsc/tests/oap/Makefile.am b/openbsc/tests/oap/Makefile.am
index e160902..3a76b11 100644
--- a/openbsc/tests/oap/Makefile.am
+++ b/openbsc/tests/oap/Makefile.am
@@ -3,7 +3,11 @@
 
 EXTRA_DIST = oap_test.ok
 
+if HAVE_LIBGTP
+if HAVE_LIBCARES
 noinst_PROGRAMS = oap_test
+endif
+endif
 
 oap_test_SOURCES = oap_test.c