harfbuzz: fix build errors on NetBSD 7.2 with gcc 4.9.

> In file included from ../test/api/test-aat-layout.c:25:0:
> ../test/api/hb-test.h: In function 'hb_test_assert_blobs_equal':
> ../test/api/hb-test.h:178:5: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
>      for (unsigned int i = 0; i < expected_length; i++)
>      ^
> ../test/api/hb-test.h:178:5: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code

(tsutsui)