spa: declare a dependency for the dbus lib

When running meson test without a preceding ninja build, we end up with
missing libraries. This somehow happened to work until recently but now
it triggers an issue with libspa-dbus.so not being available during the
valgrind test run, causing a test case to fail.

test/meson.build

@@ -73,6 +73,7 @@ test('test context',
                'test-context.c',
                'test-config.c',
                include_directories: pwtest_inc,
+               dependencies: [spa_support_dep, spa_dbus_dep],
                link_with: [pwtest_lib,
                             pipewire_module_protocol_native,
                             pipewire_module_client_node,