ffmpeg: Fix RISCV build

ffmpeg: Fix RISCV build

Issue found with LLD linker

| riscv64-yoe-linux-ld.lld: error: relocation R_RISCV_64 cannot be used against symbol '
   ↪ ff_h264_weight_pixels16_8_rvv'; recompile with -fPIC
   | >>> defined in libavcodec/riscv/h264dsp_rvv.o
   | >>> referenced by libavcodec/riscv/h264dsp_rvv.o:(ff_h264_weight_funcs_8_rvv)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>