riscv: Fix sparse warning in vendor_extensions/sifive.c

riscv: Fix sparse warning in vendor_extensions/sifive.c

sparse reports the following warning:

arch/riscv/kernel/vendor_extensions/sifive.c:11:33: sparse: sparse: symbol 'riscv_isa_vendor_ext_sifive' was not declared. Should it be static?

So as this struct is only used in this file, make it static.

Fixes: 2d147d77ae6e ("riscv: Add SiFive xsfvqmaccdod and xsfvqmaccqoq vendor extensions")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505072100.TZlEp8h1-lkp@intel.com/
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250620-dev-alex-fix_sparse_sifive_v1-v1-1-efa3a6f93846@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>