llvmpipe: fixes error: ‘enum pipe_blendfactor’ declared in lp_test_blend.c

llvmpipe: fixes error: ‘enum pipe_blendfactor’ declared  in lp_test_blend.c

error  message:
In file included from ../../src/gallium/drivers/llvmpipe/lp_test_blend.c:44:
../../src/gallium/drivers/llvmpipe/lp_bld_blend.h:47:21: error: ‘enum pipe_blendfactor’ declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
   47 |                enum pipe_blendfactor factor_src,

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19676>