Fix MIPS compile.

BUG=chromium:524885
TEST=none
TBR=turaj

Review URL: https://codereview.webrtc.org/1316843003 .

Cr-Commit-Position: refs/heads/master@{#9790}
diff --git a/webrtc/modules/audio_coding/codecs/isac/fix/source/lattice_mips.c b/webrtc/modules/audio_coding/codecs/isac/fix/source/lattice_mips.c
index 7197b15..d488bfc 100644
--- a/webrtc/modules/audio_coding/codecs/isac/fix/source/lattice_mips.c
+++ b/webrtc/modules/audio_coding/codecs/isac/fix/source/lattice_mips.c
@@ -8,6 +8,8 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <stddef.h>
+
 #include "webrtc/modules/audio_coding/codecs/isac/fix/source/settings.h"
 #include "webrtc/typedefs.h"