Add missing include.

This was previously leaked by libc++.

Test: mma
Bug: http://b/34740564
Change-Id: I7084cdb44a9b7346556c20b1b6eeebc6ebb2d500
diff --git a/drmeventlistener.cpp b/drmeventlistener.cpp
index b3e2328..4b8a463 100644
--- a/drmeventlistener.cpp
+++ b/drmeventlistener.cpp
@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+#include <assert.h>
+
 #define LOG_TAG "hwc-drm-event-listener"
 
 #include "drmeventlistener.h"