Workaround sd_bus_message_append_cmdline

Unable to get libmapper built without systemd being patched with the
sd_bus_message_append_cmdline function. Until it is pushed upstream,
this is a workaround to get the available systemd install built for unit
testing.

Change-Id: Ieca78955a4182eebd469a860c37ccaedfe75d323
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/libmapper/app.c b/libmapper/app.c
index f0d6b03..887d02f 100644
--- a/libmapper/app.c
+++ b/libmapper/app.c
@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+#include "config.h"
 #include <stdlib.h>
 #include <stdio.h>
 #include <systemd/sd-bus.h>