mgcp_client_test makefile: add update_exp target

Change-Id: I8cda082c46ab5734873cbb39b313037d86777589
diff --git a/tests/mgcp_client/Makefile.am b/tests/mgcp_client/Makefile.am
index 5cd0d8a..5ce9538 100644
--- a/tests/mgcp_client/Makefile.am
+++ b/tests/mgcp_client/Makefile.am
@@ -37,3 +37,6 @@
 	$(LIBRARY_DL) \
 	$(LIBOSMONETIF_LIBS) \
 	$(NULL)
+
+update_exp:
+	$(builddir)/mgcp_client_test >$(srcdir)/mgcp_client_test.ok 2>$(srcdir)/mgcp_client_test.err