Fix initialisers in dct-test.c.

Silences a warning when running fate on unusual platforms:
libavcodec/dct-test.c:101:21: warning: missing braces around initializer [-Wmissing-braces]
libavcodec/dct-test.c:101:21: warning: (near initialization for 'fdct_tab_arch[0]') [-Wmissing-braces]
libavcodec/dct-test.c:102:21: warning: missing braces around initializer [-Wmissing-braces]
libavcodec/dct-test.c:102:21: warning: (near initialization for 'idct_tab_arch[0]') [-Wmissing-braces]