vp9/x86: make STORE_2X2 macro local.

Prevents this assembler warning:
libavcodec/x86/vp9itxfm.asm:1208: warning: (VP9_IDCT32_1D:309)
redefining multi-line macro `STORE_2X2'

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>