ransac.c: define _POSIX_C_SOURCE for rand_r

rand_r() isn't visible by default with -std=c99. this can be changed to
_POSIX_SOURCE after -std=c99 is enabled.; the portability of rand_r()
can be addressed in a future change.

BUG=aomedia:111

Change-Id: Id540f7f4a70007f70585261814b6fb09925fb32b

av1/encoder/ransac.c

@@ -8,6 +8,7 @@
  * 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.
  */
+#define _POSIX_C_SOURCE 200112L  // rand_r()
 #include <memory.h>
 #include <math.h>
 #include <time.h>