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
1 file changed