Include <assert.h> in av1/av1_cx_iface.c

av1/av1_cx_iface.c uses the assert() macro.

Change-Id: I05680f2e477fa1c9c9ed0e26981c9ef39ded7d7d

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 3210f77..eb510eb 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -8,6 +8,8 @@
  * Media Patent License 1.0 was not distributed with this source code in the
  * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
  */
+
+#include <assert.h>
 #include <limits.h>
 #include <math.h>
 #include <stdbool.h>