spirv: Fix windows build
Looks like MSVC doesn't like VLAs:
src/compiler/spirv/spirv_to_nir.c(3879): error C2057: expected constant expression src/compiler/spirv/spirv_to_nir.c(3879): error C2466: cannot allocate an array of constant size 0 src/compiler/spirv/spirv_to_nir.c(3879): error C2133: 'srcs': unknown size
so let's use a static array size.
Fixes: 87d74311 ("spirv: Use nir_vec_scalars() to simplify matrix transpose.")