multimedia/xvid: fix build on powerpc with LLVM 13

../../src/image/ppc_asm/colorspace_altivec.c:434:1: error: address of register variable requested
MAKE_COLORSPACE_ALTIVEC_FROM_RGB(bgra_to_yv12_altivec_c, 4, 8, 2, RGB_TO_YV12_ALTIVEC, 2, 1, 0, 0)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../src/image/ppc_asm/colorspace_altivec.c:65:5: note: expanded from macro 'MAKE_COLORSPACE_ALTIVEC_FROM_RGB'

shift_consts[0] = vec_add(vec_splat_u32(12), vec_splat_u32(12));    \
^~~~~~~~~~~~

Fix similar to 0efce95397d4b53be14448a803902191ab8c1df3.