fastssim: Add stdlib.h for malloc/free

Change-Id: I4d734febc14c534dba20b67cf6bd628996cc9ab7

vpx_dsp/fastssim.c

diff --git a/vpx_dsp/fastssim.c b/vpx_dsp/fastssim.c
index 7024cbf..25f01e5 100644
--- a/vpx_dsp/fastssim.c
+++ b/vpx_dsp/fastssim.c
@@ -11,6 +11,7 @@
  *  project.
  */
 #include <math.h>
+#include <stdlib.h>
 #include <string.h>
 #include "./vpx_config.h"
 #include "./vpx_dsp_rtcd.h"