sad_test: fix compilation w/gcc 4.8.5

use a #define for kDataAlignment as it's used with DECLARE_ALIGNED
(__attribute__((aligned(n)))) and this version under CentOS is more
strict over integer constants:

../vpx_ports/mem.h:18:72: error: requested alignment is not an integer constant
 #define DECLARE_ALIGNED(n, typ, val) typ val __attribute__((aligned(n)))

Bug: webm:1690
Change-Id: I8d4661ec1c2c1b1522bdc210689715d2302c7e72
1 file changed