Include <cstring> for memset()

Change-Id: I0f4b7773ade684efdc6686afb1c69c96300edf64

test/encode_small_width_height_test.cc

diff --git a/test/encode_small_width_height_test.cc b/test/encode_small_width_height_test.cc
index 2d705a7..d74d613 100644
--- a/test/encode_small_width_height_test.cc
+++ b/test/encode_small_width_height_test.cc
@@ -15,6 +15,7 @@
 // (<= one AV1 superblock) with multiple threads. aom_codec_encode() should
 // not crash.
 
+#include <cstring>
 #include <memory>
 
 #include "aom/aomcx.h"